The reference is fs.md's per-file semantics, errors and limits in Typst, with the settings table, and says what the pane ctl's Left, Right, Up and Down do

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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+// The reference: every file the 9P control filesystem serves, what it
+// does, how it fails, and the limits. How pardes behaves on screen is the
+// guide's; recipes and traps are scripting's; how a mount cuts writes and
+// the listeners are building's.
+#import "style.typ": key, keys, btn, chord, word, tag, addr, file, cmd, doc, pairs
+
+Every native session serves 9P2000 (not .u, not .L) as a control
+filesystem, in acme's manner: panes, columns, tags and the session are
+files. Paths here are the served ones (#file("/pane/2/body")); through a
+mount they sit under the session's directory (`$m`; the scripting chapter
+says how to find it). The served #file("/README") is a
+one-screen summary of this page.
+
+= The tree <tree>
+
+```
+/README        the one-screen summary (src/fs-help.txt)
+/index         a line per pane: serial kind dirty name column
+/status        pid, version, panes
+/look /exec    write a line: a right / middle click at the active pane; read: the serials touched
+/log           the event log; write `follow` to wait for more
+/screen        the rendered screen as JSON
+/listeners     dial addresses
+/focus         the serial of the pane with the keyboard; write one to move it
+/ctl           settings and session builtins
+/commands      every builtin, one a line
+/recent        files opened lately: open|closed <path>
+/layout        a line per column, then `active <serial>`
+/tag /tagexec  the workspace tag, and a word clicked in it
+/col/<n>/      tag ctl exec of column <n>; rmdir closes an empty one
+/pane/new      open it to make a pane; read answers the serial
+/pane/<n>/     name body tag ctl addr dot limit data xdata sel dirty mark scroll
+               errors event look exec tagexec, and pty/{ctl,status,data,run} on terminals;
+               rmdir closes the pane
+/os/           the host filesystem
+/src/          the editor's sources (only with -Dembed-sources=true)
+```
+
+Panes and columns are named by serials the editor gives: stable while they
+live, never reused. Nothing is created by a listing, stat, walk or read:
+only an open of #file("/pane/new") makes a pane (so `ls -l` and `find` are
+safe), only `rmdir` of #file("/pane/<n>") or an empty #file("/col/<n>")
+removes. Tcreate is refused everywhere.
+
+#file("/index") rows are `serial kind dirty name column`, kind `text`,
+`term`, `pdf` or `image`, the name `<dir>/+New` for an unnamed scratch.
+Names may hold blanks, so split `head, col = row.rsplit(maxsplit=1)`, then
+`serial, kind, dirty, name = head.split(maxsplit=3)`. Names in
+#file("/index"), the log and a terminal's tag are escaped: a newline `\n`, a
+backslash `\\`, a byte that is not UTF-8 `\xNN`.
+
+= Rules for every file <rules>
+
+*Failure.* A write fails whenever what it asked for fails, and logs one
+`err <serial|-> <file>: <why>` record, with no `msg`. Only writes log: a
+refused open or truncation (an OTRUNC open such as `> data` after a failed
+`addr`), create or remove answers its error alone, as do a write to
+#file("pane/new") (`permission denied`) and a write on a read-only fid
+(`bad use of fid`). Errors are words (Plan 9's where pardes has none of its
+own), never a C library string. Through 9ns the kernel sees an errno 9ns
+reads from those words (cloud9's `9ns/src/nine.zig`, `enameToErrno`):
+`control message`, `invalid` or `bad ` is EINVAL (malformed input); `no such`, `not found` ENOENT; `in use` EBUSY; `no space` ENOSPC; `denied`
+EACCES; anything else, such as `no match for regexp`, `address out of range`, `Modified` or an `Edit` command pardes leaves out (`w is a sam command pardes's Edit leaves out`), EIO; no refusal reads as EOPNOTSUPP.
+The `err` record has the words; a shell sees only the errno, most often
+`Invalid argument` or `Input/output error`.
+
+Not failures: a look that finds nothing (it answers nothing and logs one
+`err`), an Edit `x` that matches nothing, #word("Undo") with nothing to
+undo (a `msg`), and a command pane's command, which ends in its own time
+with an `exit` record.
+
+*Command lines.* #file("look"), #file("exec"), #file("tagexec"), the three
+kinds of #file("ctl") and a column's #file("exec") take one command a line.
+The whole write is checked first (a control character other than a tab
+fails it all, EINVAL); then lines run in order, and a failing line fails
+the write after the lines before it took effect, as acme's ctl does. Blank
+lines are skipped. A line runs once it is whole: a write's last line runs
+without its newline, except where a mount cut the write (the building
+chapter has the mechanics); end a write with a newline when its result
+matters. An `Edit` whose `{` or `a`/`c`/`i` text is still
+open waits for the next write on that open. A line held past 1 MiB is
+refused.
+
+*Answers.* Reading #file("look"), #file("exec") or #file("tagexec") answers
+the serials the last command made, or else the pane it acted on or
+focused, one a line; nothing when it did none of these (#word("Newcol") at
+#file("/tagexec")). An open that wrote reads its own last answer; an open
+that never wrote reads the session's last. A read is a stream: once read,
+the next read on that fid is EOF until the next write (or seek to 0). With
+other clients about, write and read on one open:
+#cmd("exec 3<>$m/look; echo x >&3; cat <&3; exec 3<&-")
+
+*Snapshots.* #file("/index"), #file("/layout"), #file("/recent"),
+#file("/commands"), #file("/status"), #file("/listeners"), a read-only
+#file("ctl"), #file("/log"), #file("/screen") and a terminal's
+#file("body") freeze at the open, so one read in several chunks never
+splices two moments; open again for now. Such an open, or a #file("run"),
+#file("event") or #file("pty/data") open, takes one of 64 open records;
+past that the open fails `too many open files`.
+
+*Held reads.* A read with nothing to give yet (a followed #file("log"),
+#file("event"), #file("pty/data"), #file("pty/run") before its answer)
+waits in the editor and is answered when news comes. A second read on that
+open meanwhile fails `file in use`. A read the client flushed is dropped.
+One connection holds at most 128 reads at once; the next is refused `too many reads waiting: 128`. Every held read is on an open that keeps state,
+and those are 64 in the session (above), so 64 is the real cap on reads
+held at once, through one connection or many. A mount (9ns) is one
+connection, and it keeps answering everything else beside its held reads.
+
+*Stats.* A file whose text is kept has its real length: a pane's
+#file("body"), #file("tag"), #file("name"), #file("ctl"), #file("sel") and
+the range and flag files, the workspace's #file("tag"), the root
+#file("ctl"), #file("status"), #file("commands"), #file("README"), the
+#file("look")/#file("exec") answers, #file("focus"); #file("event") and
+#file("pty/data") the next record's, zero when none waits; #file("log")
+what an open would freeze. A view generated by each read stats 0, as
+acme's do: #file("screen"), #file("data"), #file("xdata"), #file("index"),
+#file("layout"), #file("recent"), #file("listeners"), #file("pane/new").
+Read those to the end rather than trust a length (`cat` does). The qid
+version of #file("body"), #file("data") and #file("xdata") is the pane's
+revision, so a stat sees an edit land. Modes are 0644/0666, 0444
+read-only, 0222 write-only.
+
+= look and exec <look-and-exec>
+
+A line written to #file("look") is a #btn("B3") click on it, on the line as
+written: its leading blanks are its own (`    return x` finds that
+indented line, and a diff's blank context line ` ` is a look too); only its
+newline and `\r` go. An #file("exec") line's blanks at either end are
+trimmed. The guide says what a look opens and where it looks for a relative
+path (#doc("tags", section: "looking")); here is what is particular to the
+files.
+
+- `file:12` selects line 12, newline included; `file:12:5` puts the caret
+  at line 12, byte column 5; `file:<addr>` takes any address (below),
+  *evaluated from the file's dot*: `file:/re/` finds the next match after
+  the selection, `file:0/re/` the first. `:addr` addresses the pane itself.
+  `@p<serial>:<addr>` addresses a pane by serial, a terminal's logical
+  lines too.
+- A leading `~` is the home directory (`$HOME`, else the passwd entry's;
+  `~user` that user's) here and wherever a path is typed (#file("name"),
+  #word("Save"), #word("ThemeFile"), #word("DumpDir"), #word("Restore"),
+  `pardes '~/x'`), even beside a file named `~`: write `./~` for that.
+- A path to no file is a miss, as a search that finds nothing is: said on
+  the message row and logged as an `err`, the write still answered. A file
+  that is there but will not open (no permission to read it, say) fails the
+  write, with why.
+- A plain word selects its next place after dot, wrapping (`LookWord list`
+  on the root ctl lists every place in a `+Search` pane instead). In a
+  terminal a word is always listed, rows spelled `@p3:12:5-9`.
+
+A miss logs `err <serial> look: ...` (`no match for "zzq:#3"` quoting what
+was written when nothing by that name exists; `<path>:99 has no line 99`
+for a line past a file's end, `<path> has no page 99` for a PDF's page;
+`<path>: no match for regexp` or `<path>: address out of range` when an
+address fails), opens nothing, and leaves #file("look") reading empty; the
+write succeeds. A path too long to repeat whole gives up its middle to `…`.
+
+A line written to #file("exec") is a #btn("B2") click:
+
+- A builtin word runs (#file("/commands") lists them). A builtin that needs
+  its argument (#word("Msg"), #word("Mount"), #word("Find")) written bare is
+  `wrong #args in control message "Msg"`, and so is one that takes none
+  written with one (`Config extra`).
+- A line starting with `#` is a comment, as in a shell: it runs as nothing,
+  silently, here and in every ctl.
+- The language server's words ask about a file pane's text at its cursor
+  (set it with #file("addr") and `dot=addr` first): #word("Hover") fills
+  `+Hover`; `Rename new` renames the symbol in the file and says how many
+  ranges it changed (on the message row, and so in the log) without listing
+  them, fails when there is nothing to rename, and lists the files of a
+  rename the server spreads over several in `+Search`;
+  #word("Diagnostics") and #word("Symbols") list the file's in `+Search`;
+  #word("Lspinfo") says which server serves the file and its state;
+  #word("Lspwhy") narrates the last query step by step (in `+Lsp`). The
+  write returns once the answer is in; a pane with no file is refused.
+- acme's words run as pardes's where it has one (`Put` is #word("Save"),
+  `Delete` a #word("Del") that does not ask); the rest (`Get`, `Putall`,
+  `Snarf`, `Cut`, `Paste`, `Zerox`, `Sort`, `Load`, `ID`, `Send`, `Tab`,
+  `Indent`, `Local`, `Incl`, `Abort`) are refused, `invalid: acme's Get is not a pardes builtin: ...`, never run as commands. So is a GUI-only
+  builtin (`Fonts`) on another frontend.
+- Anything else is a command line, at most 1024 bytes, run as the guide
+  says (#doc("tags", section: "command-panes")): typed into a terminal idle
+  at an empty prompt, else run in a command pane. #file("exec") reads back
+  the command pane's serial; the log says `run <serial> <line>` and `exit <serial> <N|?>`. A reused pane's #file("body") keeps every earlier run
+  above a `% <line>` row naming each command, so to take only the last
+  run's output read from after the last `% ` row:
+  #cmd("awk '/^% /{out=\"\"; next} {out = out $0 \"\\n\"} END {printf \"%s\", out}' body")
+  From a pane whose directory is gone nothing runs: `exec: <dir>: no such directory` (ENOENT).
+
+The root's #file("look") and #file("exec") act at the active pane and log as
+that pane's (with no pane at all, in the session's directory: a look opens
+its file, making a column as #word("New") does, and an exec runs its
+command there); #file("/pane/<n>/look") and #file("exec") at pane n;
+#file("/tagexec") and #file("/col/<n>/exec") click in the workspace's or
+that column's tag, run commands in the session's directory, and log as
+`-`. A pane's word (#word("Undo"), #word("Msg"), #word("Save")) is refused
+at #file("/tagexec") and a column's exec: `not a session control message "Undo": write it to pane/<n>/ctl`.
+
+A background job (`&`) outlives a command that exits on its own; its
+output goes on below `exit N` until it lets go of the pty. #word("Kill")
+(root ctl) stops the commands pardes started: bare, all; `Kill make ls`,
+those whose line starts with one of the words. For a command pane it
+signals the whole line, `&` jobs included; for a line typed into a shell
+only the foreground job (SIGTERM), and the shell decides the rest. With
+nothing running, named or not, it says `Kill: nothing running` and the
+write succeeds; words that name none of what runs fail it (`Kill: no running command has that first word`). #word("Kill") does not reach a
+REPL's code: use `sig INT` on its #file("pty/ctl").
+
+== Look paths and mounts <mounts>
+
+A look resolves the OS filesystem first, then the editor's own tree.
+Explicit paths skip that search:
+
+#pairs(
+  [`/n/os/proc/self`], [the host filesystem, served as #file("/os/proc/self")],
+  [`/n/self/pane/2/body`, `/virtual/pane/2/body`], [this session's tree, #file("/pane/2/body")],
+  [`/virtual/src/pardes.zig`], [sources embedded with `-Dembed-sources=true`, #file("/src/pardes.zig")],
+  [`/n/peer/pane/2/body`], [a mounted session: the peer's #file("/pane/2/body")],
+)
+
+`--mount=peer=work` or `Mount peer <dial>` mounts a session name, an
+absolute socket path, `unix!/path`, `tcp!IP!port` or `quic!IP!port`;
+`Unmount peer` removes it. #word("Mount") dials at once and fails if
+nothing answers (`dial failed: no answer`, `timed out`, `hung up`); with no
+dial it is `wrong #args`, and a dial that is no address `bad dial address`,
+both EINVAL. There are eight named mounts; `os` and `self` are reserved.
+#word("Unmount") refuses a mount a pane, a working directory or a pending
+Save still uses. Mounts are dumped.
+
+A session may open its own tree through a mount (a look at `$m/pane/2/body`
+from the editor serving `$m`): requests are answered on the connection's
+task while the editor waits in its syscall. Through QUIC that still hangs.
+
+= The root ctl <root-ctl>
+
+Reading #file("/ctl") gives every setting, one a line, in the words a write
+takes (`Theme orchard`, `Verbose on`, `Placement acme`, `DumpDir <dir>`,
+`Shell /bin/bash`, ...), so writing back what it reads changes nothing.
+Writes take settings and session builtins (`scope = .session` in
+`src/builtins.zig`), acting at the pane with the keyboard:
+
+- A setting written bare steps to its next value (a switch flips; so do
+  #word("Placement"), #word("BootShell"), `Crt`). A value it does not
+  take is `bad value in control message; ...` naming what it takes;
+  #file("/commands") lists them. A setting the frontend cannot show is
+  refused (`Lift is GUI-only, invalid here`).
+- #word("Newcol") makes an empty column right of the keyboard's, halving
+  it. #word("Joincol") folds the keyboard's column into the one on its
+  right (its panes go below that column's), `Joincol: no column to the right`.
+- #word("Exit") quits. While panes hold unsaved text it refuses once
+  (#doc("tags", section: "unsaved-panes")): one `unsaved <serial> <name>`
+  record per pane, then the write fails `<name>: Modified (Exit again to discard)` or `4 unsaved panes: Modified (Exit again to discard)` (EIO),
+  and the list stays in a `+Unsaved` pane. #word("Restore"), #word("Del"),
+  #word("Delcol") and a pane's `get` refuse the same way with their own
+  word.
+- #word("Dump") writes `pardes-<date>-<time>.zon` (UTC) in #word("DumpDir"),
+  logs `dump <path>`, and adds a `Restore <path>` word naming it to the
+  workspace tag (the last dump's, replacing an earlier one's).
+  `Restore [path]` replaces every pane (bare: the last dump this session
+  wrote). The Restore write is answered, then *every connection is hung
+  up*: dial again, and restart a 9ns mount. The new log has a `new` per
+  pane, `restore <path>`, then `restored <old> <new>` per pane and
+  `restoredcol <old> <new>` per column.
+- `Kill [word...]` (above), `Mount name dial`, `Unmount name`, `Theme x`.
+- `size C R` sizes a `--detach` session no frontend is attached to (160x50
+  until then): from 20x6 to 4096x4096, else `invalid size`; refused while a
+  frontend owns the size, and when a column would lose its panes' minimum
+  rows (`size: too small for the panes, each its tag and 2 rows`).
+
+A write is refused whole, before anything runs, in Plan 9's words:
+`unknown control message "X"`, `wrong #args in control message "X"`, `bad value in control message ...`, or a word of the other ctl: `not a session control message "Undo": write it to pane/<n>/ctl`, `... "Delcol": write it to col/<serial>/ctl`, `not a window control message "X": write it to /ctl`. A builtin that would open a prompt (#word("Save") on a scratch)
+fails `control message needs its argument "Save"`. A line that fails as it
+runs fails with the editor's words (`Mount: already mounted`, `Save /root/x: access denied`), changing nothing.
+
+== Settings <settings>
+
+A setting that chooses among words (`on`/`off` switches,
+#word("Placement"), #word("BootShell"), `Crt`, `Bloom`,
+`Vignette`, `Grain`, `Lift`, #word("Motion"),
+`ShaderAnimation`) steps to its next value when given bare, as its
+word clicked in a tag does. The same lines go in the startup file
+(#doc("config")).
+
+#pairs(
+  [`Theme <name>`], [`orchard`; names as #word("Themes") lists them, #word("NextColor") walks the ring (#doc("themes"))],
+  [`ThemeFile <path>`], [a `.zon` theme, relative to the config directory; reloads live when saved],
+  [`FocusTint`], [on: tint the focused pane's and column's tags],
+  [`SyntaxBold`], [off: bold syntax keywords],
+  [`Verbose`], [on: a builtin announces its name on the message row],
+  [`MessageAnimation`], [on: messages ease in and dissolve],
+  [`MessageLinger`, `MessageFall`, `MessageDissolve`], [800, 180, 150 milliseconds, at most 60000],
+  [`Placement acme|pardes`], [`acme`: where new panes go (#doc("tags", section: "new-panes"))],
+  [`BootShell keep|replace`], [`keep`; `replace` closes the untouched lone shell a dragged document lands beside],
+  [`LookWord search|list`], [`search`: a looked-at word selects its next place, or lists all in `+Search`],
+  [`Shell <name or path>`], [`$SHELL` when it is executable, else `/bin/sh`: the shell the next terminal and command pane run; a bare name is looked for in the usual bin directories, not `$PATH`; bare #word("Shell") returns to the default],
+  [`DumpDir <dir>`], [`$XDG_DATA_HOME/pardes`, else `~/.local/share/pardes`: where #word("Dump") writes; bare returns to the default],
+  [`TreeContext`], [off: sticky declaration headers in a source pane (per pane, dumped)],
+  [`TreeContextTagStyle`], [on: draw those headers in the tagline style],
+  [`LocationsConfig ...`], [the layout of #word("Grep"), search and language-server results (below)],
+  [`Wrap`, `Colors`, `Tagbottom`, `Debug`], [toggles],
+  [`Font <name>[:<size>]`, `Fonts`], [SDL and macOS only; size 8-72 (pixels in SDL, points on macOS)],
+  [`TaglineSize <1-100>`], [82: tagline face, percent; SDL and macOS],
+  [`WindowOpacity <0-100>`], [100; SDL only: everything but text and the cursor],
+  [`Ligatures`], [on; SDL only, macOS draws CoreText's own],
+  [`Pet cat|frog|off`], [off; SDL only: a sprite in the workspace tag's blank space],
+)
+
+`LocationsConfig` with no argument prints the current settings as a line
+that can be run again; with fields it changes only those:
+`LocationsConfig context:5 tscontext:on tslocations:off layout:stacked`.
+`context` (0) is the source lines shown above and below each match;
+`tscontext` (off) includes the enclosing tree-sitter declaration headers;
+`tslocations` (on) shows a location on each declaration header; `layout`
+(`stacked`) puts the location on its own line, `inline` beside the source,
+padded in groups of eight matches. An invalid field rejects the whole line;
+a repeated field's last value wins. The settings survive Dump and Restore.
+Source analysis is cached for 64 files and 64 MiB.
+
+*Effects.* Panel transitions, one at a time, running the active one again
+turning it off: #word("PanelSlide"), #word("PanelZoom"),
+#word("PanelDissolve"), #word("PanelAscii"), #word("PanelVertical"),
+#word("PanelEdges"), #word("PanelFall"), #word("PanelWave"),
+#word("PanelCurtain"), #word("PanelScramble"), #word("PanelType"); all
+start off, and the web shell has none. Scene passes (the SDL window, and
+one attached to a detached session), each at a level 0-3 (`on` is 2), all
+off: `Crt`, `Bloom`, `Vignette`, `Grain`.
+`Shader <file.glsl>` adds a Shadertoy file written for ghostty to the
+chain (`Shader off` removes it; it recompiles when saved);
+`ShaderAnimation off|on|always` says when the chain animates by itself. The
+focused pane can stand off the page: `Lift shadow|rim|auto|off`,
+`InactiveDim <percent>`, `Motion off|crisp|smooth|bouncy|playful` (default
+`smooth`), `SelectionGlow`, `HoverGlow`, `Occlusion`,
+`Parallax`, #word("JumpTrail"), #word("ChipShadow"),
+#word("ThumbFlash"), `CursorBlink`, `GripWidth <50-300>`. Most are
+the GUI's; #word("InactiveDim") works everywhere, and #word("JumpTrail"),
+#word("ChipShadow") and #word("ThumbFlash") are the terminal's. No effect
+may lower the contrast of text, the selection or a focus indicator.
+`EffectCode <effect>` lists that effect's sources under `/virtual` when the
+build embeds them.
+
+Resting the pointer on text for about 32 ms (`look_preview_delay_frames`,
+2 frames, in `src/config.zig`; `null` turns it off) tints what a
+#btn("B3") click would look at, with no other effect. A terminal's
+#word("Filter") keeps each foreground at least `tty_filter_min_contrast`
+(WCAG 1.5, `src/config.zig`) against its background.
+
+= Columns and tags <columns-and-tags>
+
+#file("/layout") has a line per column, left to right: `serial index x width current|notcurrent empty|full pane-serials...`, then `active <serial>` (the active column: where #file("pane/new") and a look put the
+next pane; `-` when none). `current` is the column with the keyboard now,
+which may differ from the active one (#doc("tags", section: "active-column")).
+#file("/index")'s last field is each pane's column serial.
+
+A session holds 16 columns (`no space for a column: 16 max`, ENOSPC), each
+at least 10 cells wide, so 16 wants a window 160 cells wide or more.
+#word("Newcol") halves the column it is run from, and one under 20 cells
+does not split (`this one is too narrow to split`): reach 16 by writing
+#word("Newcol") to the widest column's #file("exec") each time, not to the
+root's, which keeps halving the one just made. #word("Newcol") is refused
+as well when a narrower column would wrap its panes' tags onto more rows
+and leave one under its tag and two rows (`Newcol: no space for a column: the panes' tags would not fit`); a refused #word("Newcol") logs its `err`
+alone and uses up no column serial.
+
+#file("/col/<n>/ctl") takes #word("Delcol"), #word("Joincol"),
+#word("New") and #word("Tty") for that column; #file("/col/<n>/exec") is
+a click in its tag; `rmdir /col/<n>` closes an empty column (one with
+panes: ENOTEMPTY). Closing a column's last pane leaves it empty (the
+keyboard goes to its tag, #file("focus") reads empty, the log says only
+`del`); closing the session's last pane quits pardes. The log says
+`newcol <serial>` and `delcol <serial>`.
+
+#file("tag") files (#file("/tag"), #file("/col/<n>/tag"),
+#file("/pane/<n>/tag")) read the whole tag, with no newline after it. A
+pane's starts with its computed path (an image's with its mode words, a
+PDF's with its page), then the editable text. `>` replaces the editable
+text (the default words too: `echo Make > tag` leaves only `Make`) and
+drops the one newline that ends it; `>>` appends, so
+`printf ' Make' >> tag` (the blank matters; `echo` would start a second
+line). A pane tag may hold several lines; a column or workspace tag is
+one, a newline written into it becoming a space. Control characters other
+than tab, DEL, C1 controls and non-UTF-8 bytes are refused (`invalid tag text`). The editable text is at most 4096 bytes (`no space: over 4096 bytes`, ENOSPC; the log says `err <serial> tag: no space: ...`). All
+three kinds take the same checks, whole or not at all, and a `>` whose
+write is refused changes nothing: its truncation is done with the write
+that fits, or at the close (or a read) when none came. Each write stands
+on its own: in a `>` cut into several writes, those taken before a refused
+one stay in the tag; only the refused write changes nothing. A clear is an
+ordinary edit and #key("u") in the tag undoes it.
+
+= Panes <panes>
+
+*Making and closing.* Opening #file("/pane/new") makes a scratch
+`<dir>/+New` (the session's directory), and reading that open answers its
+serial: `n=$(cat $m/pane/new)`. *Each open makes another pane*; two reads
+of one open answer the same serial. It goes where a #word("New") from a
+tag would (#doc("tags", section: "new-panes")): in the active column,
+filling it if empty, else the bottom half of its last pane. A session
+holds 64 panes (`no space for a pane: 64 max`), and every pane keeps its
+tag and 2 rows (`no space for a pane in that column: each keeps its tag and 2 rows`); both are ENOSPC, for this open and for a look, exec,
+#word("New") or #word("Tty") alike. A #file("pane/new") whose column is
+full takes its rows from a pane in another column that has them, last
+column first, as acme does, and is refused only when no pane anywhere can
+give them. `rmdir /pane/<n>` closes the pane, unsaved or not.
+
+*#file("name")* reads the file name (a terminal's directory); a write
+renames the buffer (relative to the pane's directory) and marks nothing
+dirty; #word("Save") then writes under the new name. A name takes any byte
+a file name can hold, blanks, control bytes and bytes not UTF-8 included,
+so what #file("name") reads writes back as it was; refused (EINVAL) are
+only a second line and a NUL (`bad character in file name: a NUL`). Up to
+255 bytes a component. The ctl word `name x` takes all after its one
+blank; a second blank there is refused rather than read as the name's
+first byte. A directory (`/`, `~`, `foo/`) is no file name: refused,
+EISDIR (`name: /home/u is a directory, not a file`).
+
+*#file("body")* reads the text; a write appends; `>` (OTRUNC) replaces it
+all. A terminal's body is its history as plain text in logical lines
+(wrapped rows joined), frozen per open; writing it sends input to the
+child as typed keys, never a paste: no bracketed-paste marks around it,
+even when the program asked for them, so a newline in it is Enter. A PDF's
+body is the text layer of the page shown, read-only: it is no text of the
+pane's, so #file("addr"), #file("data"), #file("dot") and a `file:<addr>`
+look do not address it (a PDF's `:<n>` is its page); images and PDFs take
+no write (`this pane has no text`).
+
+*#file("sel")* reads the selected text; a write replaces it.
+*#file("errors")* is write-only: text appended to the directory's
+`+Errors` pane (logged as `msg` records when no column has room).
+
+*#file("focus")* (root): a serial written gives that pane the keyboard and
+makes its column the active one (a folded pane stays folded); `no such pane`, or `ill-formed control message` for a non-number. It reads empty
+while a column or workspace tag has the keyboard.
+
+*Pane ctl.* Reading it gives acme's status line: serial, tag length, body
+length, isdir (0), dirty, width in cells, font, tab width, undo available,
+redo available, then `current`/`notcurrent` and a REPL's id if bound. It
+takes:
+
+- the pane builtins: #word("Del") (`Del k`/`Del j`, or
+  #word("DelAbove")/#word("DelBelow"), give its rows to the pane above or
+  below), `Save [path]` (making the directories the file goes in first,
+  whichever name it writes), #word("Collapse"), #word("Undo")/#word("Redo")
+  (256 steps each), `Find pat`, `Edit ...`, `Tty [shell]` (a new terminal
+  in its directory), #word("Delcol") for its column, and
+  #word("Left")/#word("Right")/#word("Up")/#word("Down"), which give the
+  keyboard to the pane beside it that way (#key("Ctrl-w") with
+  #keys("h", "l", "k", "j")), up from a column's top pane to its tag.
+- `get`: reload from the file (refused once over unsaved edits, `<name>: Modified (get again to discard)`).
+- `lock`/`unlock` (acme's). The lock binds only clients that take it and
+  belongs to the open that wrote it: `exec 3>$pane/ctl; echo lock >&3; ...; exec 3>&-`. A `lock` another open holds fails at once, `file in use`
+  (EBUSY): retry.
+- `answer <choice>` to the question the pane asks on its notice band,
+  logged `ask <serial> <what> <choices>` (`ask 4 del k j`, `ask 4 repl a b`, `ask 4 save path`); `answer -` takes it back.
+- acme's lowercase ctl words, done by what replaces each: `name x`, `put`
+  (Save), `clean`/`dirty`, `del`, `delete` (no asking), `dot=addr`,
+  `addr=dot`, `limit=addr`, `mark`/`nomark`, `show`, `cleartag`. `dump`,
+  `dumpdir`, `font`, `menu`, `nomenu` are refused, EINVAL. These lowercase
+  words are ctl-only: written to an #file("exec"), `del` is a command line.
+
+A file changed on disk reloads by itself only when the buffer has no
+unsaved edits; otherwise it stays dirty, says `<name> changed on disk (get reloads it, Save overwrites it)`, logs `changed <serial>`, and its next
+#word("Save") warns once.
+
+== Addresses and data <addresses>
+
+#file("addr"), #file("dot") and #file("limit") read the pair of byte
+offsets they take, so copying one onto another (`cp $p/addr $p/dot`) is
+acme's `dot=addr`. A write is a pair or an address expression; a pair is
+checked as an address is, `addresses out of order` (`5 2`) or `address out of range` (past the text), never clamped. #file("addr") names where
+#file("data") reads (to the end of text) and the range #file("xdata")
+reads; #file("dot") is the selection (moving it scrolls the pane there);
+#file("limit") bounds the end of a forward search and reads empty until
+set. Truncating #file("dot") empties it, truncating #file("limit") lifts
+it.
+
+- A write to #file("data") or #file("xdata") *replaces* the #file("addr")
+  range (`>` and `>>` alike); `: > data` deletes it. Truncating
+  #file("data") is pardes's own (acme ignores OTRUNC there); only
+  truncating #file("body") empties the buffer.
+- A write leaves #file("addr") just past what it wrote, so a second
+  `echo x > data` inserts after the first: write #file("addr") before each
+  replacement. A read of #file("data")/#file("xdata") moves #file("addr")
+  past what it read.
+- #file("addr") belongs to the pane, not the client, and neither an open
+  nor a truncation resets it (acme resets on first open): each
+  `echo /re/ > addr` searches on from the last, so a find loop advances.
+  Write `0` to start at the top; searches wrap, so stop a loop when the
+  address comes back or set #file("limit").
+- A failed address leaves *no address*: #file("addr") reads empty and
+  #file("data")/#file("xdata") refuse (`no address: the last one written to addr failed`) until a standalone address (`2`, `#0`, `/re/`) is written,
+  so a missed target is never written at the old one.
+
+Addresses are sam's: `#n`, a line number, `/re/`, `?re?`, `-/re/`, `$`,
+`.`, `0`, ranges `a,b` and `a;b`, `+`/`-`. They are evaluated from the
+current address (the last written to #file("addr"), or just past the last
+#file("data") write), not from the selection. pardes adds `12:5`: line 12,
+*byte* column 5 from 1, clamped to the line's end (`12:0` is `address out of range: a column counts from 1`); it composes (`12:5,14:1`). A tool's
+character column matches only on an ASCII line; elsewhere use `12/name/`
+or `#n`. A row of #word("Recent"), `+Search` or the #word("Jumplist")
+spells a range `12:5-14:2` (through 14:2 inclusive) and #file("addr")
+takes it as such.
+
+Offsets and counts are bytes everywhere (acme counts runes), but every
+address lands on a rune boundary: `#n` or `L:C` inside a rune snaps to its
+start, a match covers the runes it touches, and a combining mark or a
+CRLF's `\r` is addressable alone.
+
+Refusals: `bad address syntax`, `no match for regexp`, `address out of range`, `addresses out of order` (`#100,#50`), `bad regular expression`.
+
+sam details kept: `$-1` is the last line when the text ends in a newline,
+else the one before; with a final newline the empty place after it is a
+line (`1` of an empty text is `#0,#0`, so `Edit 1i/x/` works on an empty
+file); `2,1` is an empty range at line 2's start; `/^/` finds the empty
+place after a final newline; a pattern that can match empty (`^`) passes
+over the match where the search starts.
+
+== Regular expressions <regexp>
+
+Patterns are mvzr's (classes, `\d\w\s`, `{m,n}`, lazy `*?`), searched as
+sam searches, line by line: `^` and `$` match at any line's start and end,
+`.` and `[^...]` never match a newline. The same code (`src/regexp.zig`)
+serves addresses, Edit, and normal mode's #key("s") and #key("S"). The
+ceiling:
+
+- The leftmost match wins, but among alternatives the first that matches,
+  not sam's longest (`/gam|gamma/` finds `gam`). mvzr keeps no submatches,
+  so Edit's `s` has no `\1`-`\9`.
+- A pattern holding `\n` runs over the whole text: there `^` may only come
+  first and `$` only just before a `\n`, else it is refused.
+- An alternation must anchor every branch with `^` or none (`^def|^ `
+  works, `^def|x` is refused: `an alternation anchors every branch with ^ or none`). A `$` does not count: `foo$|bar` is fine.
+- A class may hold non-ASCII runes (`[éa-z]`, a range up to 256 runes); a
+  wider range or a negated class with one (`[^é]`) is refused.
+- At most 512 operations (about 512 characters, counted after that
+  rewriting): `bad regular expression: longer than mvzr's 512 operations (about 512 pattern characters)`.
+- Each search has a step budget (about 300 ms; each search of an Edit `x`
+  its own): `regular expression search took too much time, gave up`. What
+  runs out is exponential backtracking (`a?` twenty times then twenty
+  `a`s) or a quadratic pattern over a very long line.
+
+== Edit <edit>
+
+`Edit <sam commands>` on a pane's #file("ctl") or #file("exec") (or the
+root's, at the active pane) runs acme's Edit on the body: addresses as
+above, commands `x y g v c a i d s p = m t u` and `{ }`. All changes are
+one undo step, applied only if every command succeeds; a failure changes
+nothing and fails the write with acme's words (`Edit: no substitution`).
+An `x` that finds nothing succeeds silently. `p` and `=` print to the
+directory's `+Errors`. Not there: `b B D e r w f X Y`, `< | >`,
+`\1`-`\9`. In `s`, `&` is the match (`\&` a literal); in `c`, `a`, `i` it
+is a literal. `y` yields the stretch before the first match too. Braces
+take a command a line, so a block goes on one open, in one write or
+several:
+#cmd("printf 'Edit ,x/foo/{\\ni/</\\na/>/\\n}\\n' > $p/ctl")
+
+== Flags and undo <flags>
+
+#file("dirty"), #file("mark") and #file("scroll") read and take `0` or
+`1`: the buffer differs from its file (a file deleted on disk counts); a
+write pushes an undo point (writing `1` pushes one now); a write scrolls
+the pane. #file("/index")'s dirty flag is #file("dirty"); a `+New` scratch
+reads 1 but holds up nothing under 100 bytes.
+
+The writes of one open of #file("data"), #file("xdata") or #file("body")
+are one undo step (so a multi-line `printf` is one), as long as no other
+client or keystroke edits the pane between them: such an edit ends the
+step, and the open's next write starts another. To make a loop of opens
+one step: `echo 1 > mark` (a point now), `echo 0 > mark`, the writes,
+`echo 1 > mark`.
+
+== event <event>
+
+Holding #file("event") open takes the pane's look and execute clicks: they
+come to the reader as records instead of acting, as do lines written to
+the pane's own #file("look")/#file("exec") (or the root's while it has the
+keyboard). A record is acme's `<origin><action><q0> <q1> <flag> <n> <text>\n`; read `n` *bytes* of text, which may hold newlines. One open
+reads a pane's #file("event") at a time, as acme's is one window's: a
+second open for reading fails `file in use` (EBUSY) until the first is
+closed.
+
+- origin: `E` a 9P write to body or tag, `F` other files and the editor's
+  own lines, `K` keyboard, `M` mouse.
+- action: `X`/`L` executed/looked in the body, `x`/`l` in the tag (offsets
+  into the whole tag, path included), `I`/`D` body text inserted/deleted,
+  `i`/`d` the tag's.
+- flag: 1 the text's first word is a builtin, 4 (look) a file name or
+  address, 8 (exec) chorded: two records follow, the argument and where it
+  came from. pardes never sends flag 2.
+- A written line, or a click in a terminal's body, has no place: `0 0`
+  with its text (`FX0 0 1 6 Msg hi`).
+
+Write a record back to have it done as the click would: `<o><a><q0> <q1>\n` acts on that range (the last record of a write needs no newline),
+and an empty one (`MX12 12`) on the word or file name a click there expands
+to; the whole record as read acts on its text (the only way for `0 0`). A
+chorded record with its two follow-ups, in one write or three, runs once
+with its argument. `I`, `D`, `i`, `d` are refused. A helper holding
+#file("event") that writes its own pane's #file("exec") gets its command
+back as a record: run it through #file("ctl") instead.
+
+== REPLs <repls>
+
+`Repl python` on a terminal's #file("ctl") (or in its tag) binds it as
+that language's REPL: a #btn("B2") click or #key("Tab") on a `.py` body
+then types the text into the REPL instead of running it; builtin words,
+tag words, `Exec <text>` and #raw("@`cmd`") words still run. #word("Repl")
+takes the languages a code fence names (ada, bash, c, c_sharp, clojure,
+cpp, css, elixir, erlang, fortran, go, haskell, html, java, javascript,
+json, kotlin, ocaml, markdown, pascal, php, powershell, python, ruby, rust,
+scala, typst, zig) and aliases such as `py` and `sh`. Its tag and
+#file("ctl") line show its id, `python-a`. `Repl -` unbinds, bare
+#word("Repl") says the binding. With several bound, the pane asks (`ask <serial> repl a b`). Bindings are not dumped.
+
+A 9P #file("exec") is never sent to a REPL. A script either writes the
+event record `MX<q0> <q1>` to the `.py` pane's #file("event") (sent as the
+click would be), or writes the REPL's #file("pty/data") itself: multi-line
+code as a bracketed paste, `\e[200~<code>\e[201~`, then `\r` in a separate
+write once the REPL has echoed the paste; a paste of more than one line
+not ending in a newline needs a second `\r`. Line by line, a blank line
+ends a Python block and Python 3.14 auto-indents each line.
+
+= Terminals <pty>
+
+Terminal panes also have #file("pty/"):
+
+- #file("pty/data"): write bytes as typed (`printf 'ls\r'`, `\x03` is
+  Ctrl-C); read the live output stream (a consuming queue shared by
+  readers, not a replay).
+- #file("pty/status"): one line, `cols rows busy`; busy is 1 while a
+  command runs or text is typed at the prompt.
+- #file("pty/ctl"): `winsize C R` (at least 2 rows, fewer refused `invalid winsize: at least 2 rows`; at most 4096 a side), `sig INT|TERM|HUP|QUIT|KILL`, `exec` (restart the shell in its directory:
+  refused on a command pane, `a command pane does not restart`; `exec: <dir>: no such directory` if it is gone; a shell that cannot start fails
+  and leaves the old one running).
+- #file("pty/run"): one line at the shell's prompt, answered on the same
+  open: #cmd("exec 3<>$m/pane/$n/pty/run; echo make >&3; cat <&3; exec 3<&-")
+  The answer's first line is the header: `exit N` then the command's output
+  (as the screen showed it: no colour, `\r` progress collapsed, trailing
+  blanks dropped; the last 64 KiB, `exit N cut M` when M bytes were left
+  out, bare `cut` when the start scrolled away or was cleared). Or: `busy: <program> is running` (bare `busy` when text is typed at the prompt),
+  `exit ?` (no status reported, not a success), `error not run` (the shell
+  refused the line, e.g. a fish syntax error), `error shell gone`, `error no prompt marks`, and on a command pane `error a command runs here, not a shell` (`error command done; not a shell` once it ended). A line written
+  before a fresh terminal's first prompt waits for it. One line per run; a
+  second line before the answer is refused.
+
+#file("pty/run") relies on the OSC 133 marks pardes injects into bash and
+fish; `exec zsh` or a continuation prompt never reports an end, so cancel
+the read. A program on the alternate screen (vim, less) leaves no output.
+A program holding the terminal (a REPL, `less`) takes no run: write to
+#file("pty/data").
+
+= The log <log>
+
+One 64 KiB ring, one record a line, kept whether or not anyone reads it.
+An open freezes it and reads to EOF. Write `follow` to that open to then
+wait for each new record (`follow new` skips the history, as `tail -n0 -f`); `tail -f` never writes `follow`, so it sees nothing new. A follower
+the ring outran reads `lost N` first. A Restore hangs the follower up: dial
+again and read from `restore <path>`.
+
+#pairs(
+  [`new <serial> <name>`, `del`, `rename`, `save`], [a pane made, closed, renamed (a terminal's too, as its shell changes directory), saved (`Save path` of a copy: `save <serial> <path>`, the path written)],
+  [`newcol <serial>`, `delcol <serial>`], [a column made or closed],
+  [`msg <serial|-> <text>`], [the editor said something (not a builtin's own name under #word("Verbose"))],
+  [`err <serial|-> <file>: <why>`], [a write was refused or failed],
+  [`run <serial> <line>`, `exit <serial> <N|?>`], [a command pane's command started and ended; also a terminal whose shell exited, before its `del`],
+  [`send <from> <to> <repl-id>`], [text went to a REPL],
+  [`ask <serial> <what> <choices>`, `answer <serial> <choice|->`], [a pane asked, and was answered (`-`: taken back, or the pane closed)],
+  [`changed <serial> [reloaded|deleted]`], [its file changed on disk (bare: under unsaved edits)],
+  [`unsaved <serial> <name>`], [a pane an Exit, Restore, Del or Delcol refused over, before the `err`],
+  [`dump <path>`, `restore <path>`], [a Dump written; a Restore, after its panes' `new`s],
+  [`restored <old> <new>`, `restoredcol <old> <new>`], [serial maps after a Restore],
+)
+
+The serial is the pane the line ran at (the active pane for the root's
+#file("look")/#file("exec")), `-` for the root #file("ctl"),
+#file("/tagexec") and column files. A record said again straight after
+itself is counted, `err 3 addr: no match for regexp (x4)`; a follower sees
+each count as a new line. A `msg` is cut at 256 bytes and an `err` reason
+at 200, ending in `…`. Control characters become spaces.
+
+= Other files <other-files>
+
+- #file("/screen"): JSON `cols`, `rows`, `cursor`, `styles`, and row-major
+  `cells` of `[grapheme, style_index]`; one frame per open. Compare a
+  cell's style through `styles`, not the index.
+- #file("/commands"): `Word [arg] root|pane|both [values] -- sentence`, one
+  a line, generated from the builtin registry (`both`: Edit, at the active
+  pane from the root).
+- #file("/recent"): up to 200 files, PDFs and images, most recent first,
+  `open <path>` or `closed <path>`; kept in `$XDG_STATE_HOME/pardes/recent`.
+  #word("Recent") shows them in a pane, an open one at its dot now and a
+  closed one at its last (a PDF's is its page); a look at a row reopens it
+  there.
+- #file("/status"): `pid`, `version`, `panes`.
+- #file("/listeners"): the session's dial addresses, a line each (the
+  building chapter has the listeners).
+- #file("/os/"): existing regular files take read, write and truncation to
+  zero; create, remove, rename and mode changes are refused as not
+  permitted (`permission denied`, EACCES through a mount); ownership is
+  synthetic. Linux v9fs's truncation `mtime` hint is accepted and dropped.
+- #file("/src/") (and #file("/shaders") on GUI builds) with
+  `-Dembed-sources=true`; `EffectCode <effect>` lists an effect's files
+  under `/virtual`.
+
+= Limits <limits>
+
+#pairs(
+  [panes], [64 (16 on the board)],
+  [columns], [16 (6 on the board), each at least 10 cells wide],
+  [rows a pane keeps], [its tag and 2],
+  [msize], [65536 (64 KiB) offered],
+  [connections], [16 Unix and TCP, 16 QUIC],
+  [opens holding state], [64],
+  [held reads a connection], [128],
+  [named mounts], [8],
+  [command line], [1024 bytes; a held line or Edit block 1 MiB],
+  [tag text], [4096 bytes],
+  [regular expression], [512 operations; a step budget per search (about 300 ms)],
+  [undo], [256 steps],
+  [log], [64 KiB ring; `msg` 256 bytes, `err` reason 200],
+  [`pty/run` output], [64 KiB],
+  [file name component], [255 bytes],
+)