1 files changed, 23 insertions, 11 deletions

diff --git a/lisp/textmodes/fill.el b/lisp/textmodes/fill.el
index 85223d14f1d..f83de9f765e 100644
--- a/lisp/textmodes/fill.el
+++ b/lisp/textmodes/fill.el
@@ -130,24 +130,36 @@ Remove indentation from each line."
                 (re-search-forward "[.?!][])}\"']*$" end t))
       (insert-and-inherit ? ))))
 
-(defun fill-context-prefix (from to)
+(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 `adapive-fill-prefix' and `adaptive-fill-function'."
+This uses the variables `adapive-fill-prefix' and `adaptive-fill-function'.
+If FIRST-LINE-REGEXP is non-nil, then when taking a prefix from the
+first line, insist it must match FIRST-LINE-REGEXP."
   (save-excursion
     (goto-char from)
     (if (eolp) (forward-line 1))
     ;; Move to the second line unless there is just one.
-    (let ((firstline (point)))
+    (let ((firstline (point))
+          ;; Non-nil if we are on the second line.
+          at-second
+          result)
       (forward-line 1)
       (if (>= (point) to)
-          (goto-char firstline)))
-    (move-to-left-margin)
-    (let ((start (point))
-          (eol (save-excursion (end-of-line) (point))))
-      (if (not (looking-at paragraph-start))
-          (cond ((and adaptive-fill-regexp (looking-at adaptive-fill-regexp))
-                 (buffer-substring-no-properties start (match-end 0)))
-                (adaptive-fill-function (funcall adaptive-fill-function)))))))
+          (goto-char firstline)
+        (setq at-second t))
+      (move-to-left-margin)
+      (let ((start (point))
+            (eol (save-excursion (end-of-line) (point))))
+        (setq result
+              (if (not (looking-at paragraph-start))
+                  (cond ((and adaptive-fill-regexp (looking-at adaptive-fill-regexp))
+                         (buffer-substring-no-properties start (match-end 0)))
+                        (adaptive-fill-function (funcall adaptive-fill-function)))))
+        (and result
+             (or at-second
+                 (null first-line-regexp)
+                 (string-match first-line-regexp result))
+             result)))))
 
 (defun fill-region-as-paragraph (from to &optional justify nosqueeze)
   "Fill the region as one paragraph.