Characters that killed a render, what may sit next to a klammer, and ref/

A character written in the ^...^ mnemonic form -- ^oe^, ^zz^, ^hello^ --
used to kill ktext outright: no message, no output, and an exit status
that looked like a crash because it was one.  Only the forms that happen
to spell hexadecimal digits escaped, and they were right by luck rather
than by reading.  Any caret form that names no character is now a located
error that quotes what was written.

A klammer may now sit inside square brackets and braces as it already
could inside parentheses, and may be followed by the punctuation that
ends a sentence: [@i word@], {@b word@}, @i word@'s.  The rule is a fixed
list of characters, because none of them can occur inside a klammer name
and so none of them is ambiguous next to a delimiter.  A letter still
cannot: "x@" is the named closing delimiter, so a letter before "@" has
two readings and no rule can separate them.

The abbreviations an author writes -- the hyphen form @i-word, a klammer
glued to its neighbouring punctuation -- are now recognised as author
conveniences only.  They are not applied to @@ definitions or @@@ system
commands, which are written by someone who needs no such shorthand and
whose forms are already whole.

Also fixed, each of them silent: an exported shell function in the
environment broke every @eval in the document; a line break written with
@nl was destroyed when it passed through a klammer that generates a
table, printing as \textbackslash{}newline in the finished PDF; and
"kdesc --coverage" shortened its file column by comparing the spelling of
a path rather than the path, so a klammer set reached by a symbolic link
showed a full path where its neighbours showed a short one.

New in the distribution: ref/, a small klammerset that describes the
Klammermachine's own tables in Klammertext.  It currently provides the
character tables -- the diacritics and the extended Latin letters -- as
klammers that render to any target, so the documentation of what can be
written cannot drift from what the machine accepts.  It is early, and
will grow.

The Klammertext taxonomy now has a section on targets, describes the
@@@klammerset system command alongside the other three, and states the
precedence of "^" -- what a caret means before a letter, a digit, a
punctuation character, and a quote, in one ordered list.

(from dev 6594c7ff8c1f)

sks/target/txt_util.py

@@ -6,11 +6,11 @@ hang anything on: a block is set off from the prose around it by being
 INDENTED, and by having laid itself out before the target sees it.  Two
 consequences run through this module:
 
-  * The indent is written with "~", the non-breaking space, never with a
-    real space.  Eval::eval TRIMS a klammer's result, so leading spaces
-    never survive to the output; and the txt target's justification pass
-    treats a paragraph beginning with " " or "~" as already laid out and
-    leaves it alone (phases.txt_justify_blocks).  So the same character
+  * The indent is written with the NBSP MARKER, never with a real space.
+    Eval::eval TRIMS a klammer's result, so leading spaces never survive
+    to the output; and the txt target's justification pass treats a
+    paragraph beginning with a space or that marker as already laid out
+    and leaves it alone (phases.txt_justify_blocks).  So the same token
     both survives and declares "do not re-flow this".
 
   * A block takes the indent out of its own measure.  The page has
@@ -25,11 +25,26 @@ import sys
 import textwrap
 sys.path.append(f'{os.environ.get("KLAMMERTEXT_HOME")}/sks/kutil')
 import kutil
+import klammer_base
+import phases
 
 # The non-breaking space marker, resolved to a real space at the end of
-# phases.txt_justify_blocks.  Named here rather than written as "~" at
-# each use, since it is the whole mechanism of this module.
-NBSP = "~"
+# phases.txt_justify_blocks.  Stated once there, with the other txt
+# markers, and referred to rather than spelled out here.
+NBSP = phases.txt_nbsp_marker()
+
+def visible_length(s):
+    """How many columns s will occupy once the markers in it resolve.
+
+    len() is the wrong measure for anything laid out: a KTESC marker is
+    fourteen characters and one glyph, and an nbsp marker is thirteen and
+    one space.  Measuring the unresolved text sizes a column to a width
+    the reader never sees -- and then pads to it, so the padding is wrong
+    twice.  (The same fault as abbreviate() in mac/util.cpp, which counted
+    escape sequences as visible width, and the reason escape resolution
+    runs BEFORE the txt phases.)
+    """
+    return len(klammer_base.unescape_ktesc(s).replace(NBSP, " "))
 
 def indent_text(text, indent):
     """Every line of text prefixed with `indent` columns of NBSP."""