Improve commentary in composite.el

* lisp/composite.el (compose-gstring-for-graphic)
(compose-gstring-for-terminal): Add comments that explain
Unicode General Category mnemonics in human-readable terms.
(Bug#14461)

diff --git a/lisp/composite.el b/lisp/composite.el
index e50e5d381ec92736434b1b0171e581dcaacfefc3..76722f93c4129191b8b85ea0392a2d11aa19ef2a 100644 (file)
--- a/lisp/composite.el
+++ b/lisp/composite.el
@@ -554,6 +554,11 @@ All non-spacing characters have this function in
      ;; This sequence doesn't start with a proper base character.
      ((memq (get-char-code-property (lgstring-char gstring 0)
                                    'general-category)
+            ;; "Improper" base characters are of the following general
+            ;; categories:
+            ;;   Mark (nonspacing, combining, enclosing)
+            ;;   Separator (space, line, paragraph)
+            ;;   Other (control, format, surrogate)
            '(Mn Mc Me Zs Zl Zp Cc Cf Cs))
       nil)
 
@@ -646,6 +651,7 @@ All non-spacing characters have this function in
                          de (+ de yoff)))
                   ((and (= class 0)
                         (eq (get-char-code-property (lglyph-char glyph)
+                                                     ;; Me = enclosing mark
                                                     'general-category) 'Me))
                    ;; Artificially laying out glyphs in an enclosing
                    ;; mark is difficult.  All we can do is to adjust
@@ -771,7 +777,8 @@ prepending a space before it."
                                            'general-category)
                    'Cf)
                (progn
-                 ;; Compose by replacing with a space.
+                 ;; Compose Cf (format) control characters by
+                 ;; replacing with a space.
                  (lglyph-set-char glyph 32)
                  (lglyph-set-width glyph 1)
                  (setq i (1+ i)))