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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-# Filesystem
-
-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.
-This page is the reference for what each file does. The served
-[`/README`](../src/fs-help.txt) is its one-screen summary.
-
-## Connecting
-
-The socket is `$XDG_RUNTIME_DIR/pardes-9p-<name>.sock` (else under
-`~/.local/state/pardes`), `<name>` being the pid, the `--detach=NAME`, or
-`--9p=<name>`. A socket in the runtime directory is also posted in the 9P
-registry as `$XDG_RUNTIME_DIR/9p/pardes/<name>` (a symlink to the socket;
-[cloud9.md](cloud9.md#the-posted-9p-registry)).
-
-Pane shells get `PARDES_PID` (the editor's pid), `PARDES_9P` (the socket)
-and `PARDES_PANE` (their pane's serial).
-
-**Forwarding.** `pardes FILE` run in a pane (a live `PARDES_PID`, with
-`PARDES_9P` and `PARDES_PANE`) writes FILE to that pane's `look` and returns
-at once, as acme's `B` does. A FILE not there yet (`pardes notes/new.txt`)
-opens a new pane named for it, empty, and its Save creates the file, making
-its directories first when they are not there either. A session that
-answers never gets a nested editor in its pane: what it refuses (a bad
-name, a pane it has not) is printed, `pardes: <file>: <why>`, and the
-launch exits 1, leaving no pane behind. A missing `PARDES_9P`/`PARDES_PANE`,
-or a session that does not answer, starts a separate editor instead. Bare
-`pardes` in a pane refuses and names `--nested`.
-
-`--wait` (`-w`) returns when the pane that shows FILE is deleted (exit 0) or
-the session goes away (exit 1), as acme's `E` does. Use
-`EDITOR='pardes --wait'` (`GIT_EDITOR` follows `EDITOR`), so fish's Ctrl-O,
-`git commit` and `crontab -e` read the file after you close its pane.
-`--nested` runs a separate session whose shells do not forward to it.
-
-**Clients.**
-
-```sh
-9p -a "unix!$PARDES_9P" read index                     # plan9port, no mount
-9ns --unix "$PARDES_9P" -- sh -c 'cat "$NINE_MOUNT/index"'   # private mount
-9ns --mntgen                                           # the whole registry at /mnt/9p
-```
-
-Under `9ns --unix` the session is `$NINE_MOUNT` itself and exists only inside
-that command. Under `9ns --mntgen` every posted session is
-`$NINE_MOUNT/pardes/<pid or NAME>/`; take the name from `$PARDES_9P`. A dead
-session's entry stays listed and answers `Input/output error`, so name the
-session rather than globbing. `Tty9p` gives one pane's shell a kernel mount
-at `$PARDES_MOUNT` ([v9fs.md](v9fs.md)).
-
-plan9port's `9p write` always opens with OTRUNC, so `echo x | 9p write
-pane/3/body` replaces the whole body where acme would append. Append with
-`>>` through a mount.
-
-For [Linux v9fs](https://www.kernel.org/doc/html/latest/filesystems/9p.html)
-use `version=9p2000,cache=none,access=any`, `trans=unix` (or `trans=tcp`
-with `port=`), `uname`, `dfltuid` and `dfltgid` for the local user, and an
-empty `aname`.
-
-**Listeners.** `--9p-tcp='tcp!127.0.0.1!5640'` adds TCP;
-`--9p-quic='quic!127.0.0.1!5641'` adds QUIC (build with `-Dquic=true`,
-OpenSSL 3.6+; ALPN `pardes-9p`, an ephemeral TLS identity, no peer
-verification). Addresses are numeric IPv4/IPv6; port 0 picks one; `/listeners`
-reads them back. Every connection has full session access, `/os` included,
-and TCP is unencrypted: use loopback. Unix and TCP share 16 connection slots;
-a 17th client's Tversion gets `too many connections` (and the log
-`err - 9p: too many connections (N turned away)`). QUIC has 16 of its own.
-Plan9port and v9fs need a userspace bridge for QUIC.
-
-**Look paths and mounts.** Look resolves the OS filesystem first, then the
-editor's own tree. Explicit paths skip that search:
-
-| Look path | Meaning | Served path |
-|---|---|---|
-| `/n/os/proc/self` | the host filesystem | `/os/proc/self` |
-| `/n/self/pane/2/body`, `/virtual/pane/2/body` | this session's tree | `/pane/2/body` |
-| `/virtual/src/pardes.zig` | sources embedded with `-Dembed-sources=true` | `/src/pardes.zig` |
-| `/n/peer/pane/2/body` | a mounted session | the peer's `/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. Mount dials at once and fails if nothing answers (`dial failed:
-no answer`, `timed out`, `hung up`). There are eight named mounts; `os` and
-`self` are reserved. 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 tree
-
-```
-/README        the one-screen guide (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 `/pane/new` makes a pane (so `ls -l` and `find` are safe),
-only `rmdir` of `/pane/<n>` or an empty `/col/<n>` removes. Tcreate is
-refused everywhere.
-
-`/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 `/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
-
-**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
-`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 `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`, `e`, `r`, `|`: `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, `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.** `look`, `exec`, `tagexec`, the three kinds of `ctl` and a
-column's `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 mount cuts a big
-write into pieces of at most one message (msize 64 KiB, 65536 negotiated,
-less the header), and
-each line runs once it is whole. A write that does not fill its message is
-whole, so its last line runs even without a newline (`printf Save > exec`),
-unless it is a multiple of 4096 bytes: that is where a writer's buffer (stdio,
-a mount's page cache) filled and cut a line, so its tail waits for the next
-write or the close. An `Edit` whose `{` or
-`a`/`c`/`i` text is still open waits for the next write on that open. A
-line held to the close (a 4096-multiple write's tail, an `Edit` block never
-ended) runs there, and its failure is only in the log, as its `err` record
-(``err <serial> ctl: unmatched `{'`` or `a, c or i text not ended by a .
-line`): the close itself reports no error, and the write that sent it had
-already succeeded. A script that needs a line's result ends the write with
-a newline, so the line runs, and fails, with its write. A line held past
-1 MiB is refused.
-
-**Answers.** Reading `look`, `exec` or `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 (`Newcol` at `/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:
-`exec 3<>$m/look; echo x >&3; cat <&3; exec 3<&-`.
-
-**Snapshots.** `/index`, `/layout`, `/recent`, `/commands`, `/status`,
-`/listeners`, a read-only `ctl`, `/log`, `/screen` and a terminal's `body`
-freeze at the open, so one read in several chunks never splices two moments;
-open again for now. Such an open, or a `run`, `event` or `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 `log`, `event`,
-`pty/data`, `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. Through a FUSE mount bash's
-`read -t` cannot time out: wrap the loop in `timeout N`.
-
-**Stats.** A file whose text is kept has its real length: a pane's
-`body`, `tag`, `name`, `ctl`, `sel` and the range and flag files, the
-workspace's `tag`, the root `ctl`, `status`, `commands`, `README`, the
-`look`/`exec` answers, `focus`; `event` and `pty/data` the next record's,
-zero when none waits; `log` what an open would freeze. A view generated by
-each read stats 0, as acme's do: `screen`, `data`, `xdata`, `index`,
-`layout`, `recent`, `listeners`, `pane/new`. Read those to the end rather
-than trust a length (`cat` does). The qid
-version of `body`, `data` and `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
-
-A line written to `look` is a right 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 `exec` line's blanks at either end are trimmed.
-
-- a path opens the file (the pane already showing it, if any); `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. A leading `~`
-  is the home directory ($HOME, else the passwd entry's; `~user` that
-  user's) here and wherever a path is typed (`name`, `Save`, `ThemeFile`,
-  `DumpDir`, `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 relative path is resolved where the click was, then where you have
-  been: first against the looking pane's own directory, then against the
-  directory of each pane in the jump list, most recent first, each
-  directory tried once; the first that names a file (or a pane open on
-  that path) wins. A pane never visited (not in the jump list) is not
-  searched. `./x` and `../x` are the looking pane's directory's alone.
-- `@p<serial>:<addr>` addresses a pane by serial, a terminal's logical lines
-  too.
-- a directory types `ls` into a terminal idle there, else opens one there.
-- a URL opens in the browser.
-- in a diff pane, a whole line of the diff is a look at the `path:line` it
-  names, as a right click on its first column is
-  ([tags.md](tags.md#reviewing-diffs)).
-- 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 `look` reading empty; the write
-succeeds. A path too long to repeat whole gives up its middle to `…`.
-
-A line written to `exec` is a middle click:
-
-- a builtin word runs (`/commands` lists them): `Save`, `Del`, `New`, `Tty
-  [shell]`, `Msg text`, `Find`, `Grep`, `Edit ...`, `Mount`, ...
-  A builtin that needs its argument (`Msg`, `Mount`, `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 `addr` and `dot=addr` first): `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 /log) without listing them; one
-  that finds nothing to rename fails; a rename the server spreads over
-  other files lists them in `+Search`,
-  `Diagnostics` and `Symbols` list the file's in `+Search`, `Lspinfo` says
-  which server serves the file and its state, and `Lspwhy` narrates the
-  last query step by step (in `+Lsp`), to tell why it found nothing. 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 `Save`, `Delete`
-  a `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. At a terminal idle
-  at an empty prompt it is typed into that shell. Anywhere else it runs as
-  a **command pane**: a terminal running the root ctl's `Shell` (`$SHELL`,
-  else `/bin/sh`) with `-c` and the line, in the pane's directory, with job
-  control on. Its tag reads `<dir> (<line>) running`, then `exit N`; the log
-  says `run <serial> <line>` and `exit <serial> <N|?>`; `exec` reads back its
-  serial. A typo ends `exit 127`. The directory's next command reuses a
-  finished command pane, below what it showed; one still running gets a
-  second pane. A reused pane's `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:
-  `awk '/^% /{out=""; next} {out = out $0 "\n"} END {printf "%s", out}' body`.
-  From a column's or the workspace's tag (or `/tagexec`, `col/<n>/exec`)
-  a command always runs as a command pane, in the session's directory. From a pane whose directory is gone nothing runs: `exec:
-  <dir>: no such directory` (ENOENT).
-
-The root's `look` and `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 `New` does, and an exec runs its command there);
-`/pane/<n>/look` and `exec` at pane n; `/tagexec` and `/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 (`Undo`, `Msg`, `Save`) is refused
-at `/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. `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`). Kill does not reach a REPL's code: use `sig INT` on
-its `pty/ctl`.
-
-## The root ctl
-
-Reading `/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
-  `Placement`, `BootShell`, `Crt`). A value it does not take is `bad value in
-  control message; ...` naming what it takes; `/commands` lists them. A
-  setting the frontend cannot show is refused (`Lift is GUI-only, invalid
-  here`).
-- `Newcol` makes an empty column right of the keyboard's, halving the active
-  column. `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`.
-- `Exit` quits. It refuses once while panes hold unsaved text: 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. The same word
-  again with nothing edited since discards; after more editing it refuses
-  again, naming only the panes edited since. `Restore`, `Del`, `Delcol` and
-  a pane's `get` refuse the same way with their own word. A `+New` scratch
-  under 100 bytes, or a command's output, is never asked about.
-- `Dump` writes `pardes-<date>-<time>.zon` (UTC) in `DumpDir`
-  ([config.md](config.md#dumps)), 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. Undo history, the jump list and REPL bindings are not restored; a command pane
-  comes back showing how it ended (`exit ?` if it was running) and does not
-  run again.
-- `Kill [word...]` (above), `Mount name dial`, `Unmount name`, `Theme x`,
-  `size <cols> <rows>`.
-- `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 (`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.
-
-## Columns and tags
-
-`/layout` has a line per column, left to right: `serial index x width
-current|notcurrent empty|full pane-serials...`, then `active <serial>`
-(acme's activecol: where `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. `/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. `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 `Newcol` to the
-widest column's `exec` each time, not to the root's, which keeps halving the
-one just made. `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 `Newcol` logs its `err` alone and uses up no column serial.
-
-`/col/<n>/ctl` takes `Delcol`, `Joincol`, `New` and `Tty` for that column;
-`/col/<n>/exec` is a click in its tag; `rmdir /col/<n>` closes an empty column
-(one with panes: ENOTEMPTY). A column may be empty, as in acme: `Newcol`
-makes one, and closing its last pane leaves it (the keyboard goes to its
-tag, `focus` reads empty, the log says only `del`). Closing the session's
-last pane quits pardes. The log says `newcol <serial>` and `delcol <serial>`.
-
-`tag` files (`/tag`, `/col/<n>/tag`, `/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 `u` in the tag undoes it. [tags.md](tags.md) covers tags on screen.
-
-## Panes
-
-**Making and closing.** Opening `/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 acme's makenewwindow puts a window
-([tags.md](tags.md#where-new-panes-go)): 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, `New` or `Tty` alike. A
-`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.
-
-**`name`** reads the file name (a terminal's directory); a write renames the
-buffer (relative to the pane's directory) and marks nothing dirty; `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 `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`).
-
-**`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 `addr`, `data`, `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`).
-
-**`sel`** reads the selected text; a write replaces it. **`errors`** is
-write-only: text appended to the directory's `+Errors` pane (logged as
-`msg` records when no column has room).
-
-**`focus`** (root): a serial written gives that pane the keyboard and makes
-its column active (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: `Del` (`Del k`/`Del j`, or `DelAbove`/`DelBelow`, give
-  its rows to the pane above or below), `Save [path]` (making the
-  directories the file goes in first, whichever name it writes), `Collapse` (fold),
-  `Undo`/`Redo` (256 steps each), `Find pat`, `Edit ...`, `Tty [shell]`
-  (a new terminal pane in its directory), and the column words acting on
-  its column: `Delcol`, `Left`/`Right`/`Up`/`Down`.
-- `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 `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 `Save`
-warns once.
-
-### Addresses and data
-
-`addr`, `dot` and `limit` read the pair of byte offsets they take, so
-copying one onto another (`cp $p/addr $p/dot`) is acme's `dot=addr`. A
-write is a pair or an address expression; a pair is checked as an address
-is, `addresses out of order` (`5 2`) or `address out of range` (past the
-text), never clamped. `addr` names where `data` reads
-(to the end of text) and the range `xdata` reads; `dot` is the selection
-(moving it scrolls the pane there); `limit` bounds the end of a forward
-search and reads empty until set. Truncating `dot` empties it, truncating
-`limit` lifts it.
-
-- A write to `data` or `xdata` **replaces** the `addr` range (`>` and `>>`
-  alike); `: > data` deletes it. Truncating `data` is pardes's own (acme
-  ignores OTRUNC there); only truncating `body` empties the buffer.
-- A write leaves `addr` just past what it wrote, so a second `echo x > data`
-  inserts after the first: write `addr` before each replacement. A read of
-  `data`/`xdata` moves `addr` past what it read.
-- `addr` belongs to the pane, not the client, and neither an open nor a
-  truncation resets it (acme resets on first open): each `echo /re/ > addr`
-  searches on from the last, so a find loop advances. Write `0` to start
-  at the top; searches wrap, so stop a loop when the address comes back or
-  set `limit`.
-- A failed address leaves **no address**: `addr` reads empty and `data`/
-  `xdata` refuse (`no address: the last one written to addr failed`) until
-  a standalone address (`2`, `#0`, `/re/`) is written, so a missed target is
-  never written at the old one.
-
-Addresses are sam's: `#n`, a line number, `/re/`, `?re?`, `-/re/`, `$`,
-`.`, `0`, ranges `a,b` and `a;b`, `+`/`-`. They are evaluated from the
-current address (the last written to `addr`, or just past the last `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 Recent, `+Search` or the Jumplist spells a range `12:5-14:2` (through
-14:2 inclusive) and `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: `$-1` is the last line when the text ends in a newline,
-else the one before; with a final newline the empty place after it is a line
-(`1` of an empty text is `#0,#0`, so `Edit 1i/x/` works on an empty file);
-`2,1` is an empty range at line 2's start; `/^/` finds the empty place after
-a final newline; a pattern that can match empty (`^`) passes over the match
-where the search starts.
-
-### Regular expressions
-
-Patterns are [mvzr](https://github.com/mnemnion/mvzr)'s (classes, `\d\w\s`,
-`{m,n}`, lazy `*?`), searched as sam searches, line by line: `^` and `$`
-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
-`s` and `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 <sam commands>` on a pane's `ctl` or `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:
-
-```sh
-printf 'Edit ,x/foo/{\ni/</\na/>/\n}\n' > $p/ctl
-```
-
-### Flags and undo
-
-`dirty`, `mark` and `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. `/index`'s dirty
-flag is `dirty`; a `+New` scratch reads 1 but holds up nothing under 100
-bytes.
-
-The writes of one open of `data`, `xdata` or `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
-
-Holding `event` open takes the pane's Look and Exec clicks: they come to the
-reader as records instead of acting, as do lines written to the pane's own
-`look`/`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 `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 `event` that writes its own pane's `exec` gets its command back as a
-record: run it through `ctl` instead.
-
-### REPLs
-
-`Repl python` on a terminal's `ctl` (or in its tag) binds it as that
-language's REPL (names as a code fence spells them: `py`, `sh`, ...); its
-tag and `ctl` line show its id, `python-a`. `Repl -` unbinds, bare `Repl`
-says the binding. A middle click or the execute key on a `.py` body then
-types the text into the REPL instead of running it; builtin words, tag
-words, `Exec <text>` and @`cmd` words still run. With several bound, the
-pane asks (`ask <serial> repl a b`). Bindings are not dumped.
-
-A 9P `exec` is never sent to a REPL. A script either writes the event record
-`MX<q0> <q1>` to the `.py` pane's `event` (sent as the click would be), or
-writes the REPL's `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
-
-Terminal panes also have `pty/`:
-
-- `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).
-- `pty/status`: one line, `cols rows busy`; busy is 1 while a command runs or
-  text is typed at the prompt.
-- `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).
-- `pty/run`: one line at the shell's prompt, answered on the same open:
-
-  ```sh
-  exec 3<>$m/pane/$n/pty/run; echo make >&3; cat <&3; exec 3<&-
-  ```
-
-  The answer's first line is the header: `exit N` then the command's output
-  (as the screen showed it: no colour, `\r` progress collapsed, trailing
-  blanks dropped; the last 64 KiB, `exit N cut M` when M bytes were left
-  out, bare `cut` when the start scrolled away or was cleared). Or: `busy:
-  <program> is running` (bare `busy` when text is typed at the prompt),
-  `exit ?` (no status reported, not a success), `error not run` (the shell
-  refused the line, e.g. a fish syntax error), `error shell gone`, `error no
-  prompt marks`, and on a command pane `error a command runs here, not a
-  shell` (`error command done; not a shell` once it ended). A line written
-  before a fresh terminal's first prompt waits for it. One line per run;
-  a second line before the answer is refused.
-
-`pty/run` relies on the OSC 133 marks pardes injects into bash and fish;
-`exec zsh` or a continuation prompt never reports an end, so cancel the
-read. A program on the alternate screen (vim, less) leaves no output. A
-program holding the terminal (a REPL, `less`) takes no run: write to
-`pty/data`.
-
-## /log
-
-One 64 KiB ring, one record a line, kept whether or not anyone reads it. An
-open freezes it and reads to EOF. Write `follow` to that open to then wait
-for each new record (`follow new` skips the history, as `tail -n0 -f`);
-`tail -f` never writes `follow`, so it sees nothing new.
-
-```sh
-exec 3<>$m/log; echo follow >&3; while read -r line <&3; do ...; done
-```
-
-A follower the ring outran reads `lost N` first. A Restore hangs the
-follower up: dial again and read from `restore <path>`.
-
-| record | when |
-|---|---|
-| `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 `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
-`look`/`exec`), `-` for the root `ctl`, `/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
-
-- `/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.
-- `/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).
-- `/recent`: up to 200 files, PDFs and images, most recent first, `open
-  <path>` or `closed <path>`; kept in `$XDG_STATE_HOME/pardes/recent`.
-  `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.
-- `/status`: `pid`, `version`, `panes`.
-- `/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.
-- `/src/` (and `/shaders` on GUI builds) with `-Dembed-sources=true`;
-  `EffectCode <effect>` lists an effect's files under `/virtual`.
-
-## Limits
-
-| | |
-|---|---|
-| 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+TCP, 16 QUIC |
-| opens holding state | 64 |
-| 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 (~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 |
-
-## Source and tests
-
-`src/ninep/`: `tree.zig` (nodes, dispatch), `pane.zig`, `ctl.zig`,
-`cols.zig`, `addr.zig`, `pty.zig`, `events.zig` (event and log), `screen.zig`,
-`sources.zig`. The engine is cloud9's `fs.Server` (`src/9p.zig`); transports
-are in `src/9p_io.zig`; `src/fs.zig` holds host access, mounts and
-resolution. `zig build fs-test` drives real sessions with `test/ninep.py`;
-`zig build 9p-test` checks the engine budgets.