A line address past the last newline is sam's empty line there

Forward line addresses followed acme's number(), which refuses the empty
line after a text's last newline, so 1 of an empty buffer and 2 of "a\n"
were out of range and Edit 1i/header/ failed on an empty file. They now
follow sam's lineaddr (editors/sam/address.c:180-215): #0,#0 and #2,#2.

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

1 files changed, 6 insertions, 0 deletions

diff --git a/docs/fs.md b/docs/fs.md
index d1be9f6a..8397ed88 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -483,6 +483,12 @@ pane) and `exec` alike; an unclosed group is refused (``unmatched `{'``).
 bash's builtin `printf` writes a line at a time, so write a block with a
 heredoc or `env printf`.
 
+A line number counts newlines as sam's lineaddr does (editors/sam/
+address.c:180), so the empty line just past a text's last newline is an
+address: `1` of an empty text is `#0,#0`, `2` of `a\n` is `#2,#2`, and
+`Edit 1i/header/` on an empty file inserts; a line past that is `address
+out of range`.
+
 `line:col` is a pardes extension to sam's addresses, the spelling Look
 takes in `file:12:5`: `12:5` is the point at line 12, column 5, and it
 composes like any simple address (`12:5,14:1`, `12:5+#3`). The column is