matches are put).
REGEXP is a regular expression matching a definition construct
which is to be displayed in the menu. REGEXP may also be a
-function, called without arguments. It is expected to search
-backwards. It must return true and set `match-data' if it finds
-another element.
+function of no arguments. If REGEXP is a function, it is
+expected to search backwards, return non-nil if it finds a
+definition construct, and set `match-data' for that construct.
INDEX is an integer specifying which subexpression of REGEXP
matches the definition's name; this subexpression is displayed as
the menu item.