1 files changed, 27 insertions, 7 deletions

diff --git a/lisp/textmodes/fill.el b/lisp/textmodes/fill.el
index e4bdb9db65e..f8eb668a83b 100644
--- a/lisp/textmodes/fill.el
+++ b/lisp/textmodes/fill.el
@@ -185,6 +185,15 @@ Remove indentation from each line."
       ;; of the line and wants it to stay at the end of the line.
       (insert-before-markers-and-inherit ? )))))
 
+(defun fill-common-string-prefix (s1 s2)
+  "Return the longest common prefix of strings S1 and S2, or nil if none."
+  (let ((cmp (compare-strings s1 nil nil s2 nil nil)))
+    (if (eq cmp t) 
+        s1
+      (setq cmp (1- (abs cmp)))
+      (unless (zerop cmp)
+        (substring s1 0 cmp)))))
+    
 (defun fill-context-prefix (from to &optional first-line-regexp)
   "Compute a fill prefix from the text between FROM and TO.
 This uses the variables `adaptive-fill-regexp' and `adaptive-fill-function'
@@ -233,13 +242,24 @@ act as a paragraph-separator."
                        ;; If the second line prefix is whitespace, use it.
                        (string-match "\\`[ \t]+\\'" second-line-prefix))
                    second-line-prefix
-                 ;; If the second line has the first line prefix,
-                 ;; plus whitespace, use the part that the first line shares.
-                 (if (string-match (concat "\\`"
-                                             (regexp-quote first-line-prefix)
-                                             "[ \t]*\\'")
-                                   second-line-prefix)
-                     first-line-prefix)))
+
+                 ;; If using the common prefix of first-line-prefix
+                 ;; and second-line-prefix leads to problems, consider
+                 ;; to restore the code below that's commented out,
+                 ;; and document why a common prefix cannot be used.
+                 
+;                ;; If the second line has the first line prefix,
+;                ;; plus whitespace, use the part that the first line shares.
+;                (if (string-match (concat "\\`"
+;                                            (regexp-quote first-line-prefix)
+;                                            "[ \t]*\\'")
+;                                  second-line-prefix)
+;                    first-line-prefix)))
+
+                 ;; Use the longest common substring of both prefixes,
+                 ;; if there is one.
+                 (fill-common-string-prefix first-line-prefix
+                                            second-line-prefix)))
         ;; If we get a fill prefix from a one-line paragraph,
         ;; maybe change it to whitespace,
         ;; and check that it isn't a paragraph starter.