A column can be empty, as acme's can; new panes go where acme's makenewwindow puts them

Newcol makes an empty column; closing a column's last pane leaves it empty with
the keyboard on its tag; Delcol and Joincol alone take a column away; the
session's last pane closing quits. The +New stand-in and replaceStillborn /
stillborn_joiner are gone. Every new pane goes through exec.placeNew, which
follows acme's makenewwindow (active column, empty column whole, blank space,
halving the biggest) or, with Placement pardes, the old rules. ColumnTags is
gone: column tags are always shown. A grip drag shows acme's box cursor.

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

1 files changed, 7 insertions, 2 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 98a8638d..eaaedf02 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -69,8 +69,13 @@ for any process, and `$NINE_MOUNT/pardes/NAME/` a `--detach=NAME` session's.
 9ns exports `$NINE_MOUNT` to everything it starts, so a script checks that
 variable to know the mount is there, and takes the name from `$PARDES_9P`
 (`pardes-9p-<pid or NAME>.sock`). A new pane made through `pane/new` is a scratch named
-`<dir>/+New` until it is given a name, and closing a column's last pane
-leaves such a `+New` in its place (`Delcol` closes the column).
+`<dir>/+New` until it is given a name. A column may hold no pane, as in acme:
+`Newcol` makes one empty, and closing a column's last pane leaves it empty
+with its tag holding the keyboard (`focus` reads empty) and logs only the
+`del`. `pane/new` places its pane as acme's makenewwindow(nil) does: in the
+active column, filling it when it is empty, else taking the bottom half of its
+last pane ([where new panes go](tags.md#where-new-panes-go)). `Delcol` closes the column. Closing the
+session's last pane quits pardes; see [tags](tags.md#empty-columns).
 
 For [Linux v9fs](https://www.kernel.org/doc/html/latest/filesystems/9p.html),
 use `version=9p2000,cache=none,access=any` and `trans=unix`, or `trans=tcp`