-/* Handle ACL variables, which are not in the table because their number may
-vary depending on a build-time setting. If the variable's name is not of the
-form acl_mddd or acl_cddd, where the d's are digits, fall through to look for
-other names that start with acl_. */
+/* Handle ACL variables, whose names are of the form acl_cxxx or acl_mxxx.
+Originally, xxx had to be a number in the range 0-9 (later 0-19), but from
+release 4.64 onwards arbitrary names are permitted, as long as the first 5
+characters are acl_c or acl_m (this gave backwards compatibility at the
+changeover). There may be built-in variables whose names start acl_ but they
+should never start acl_c or acl_m. This slightly messy specification is a
+consequence of the history, needless to say.