+ * Many of the basic nonterminals used for argument list scanning come
+ * in a "normal" and a "closed" flavor. A closed expression is one
+ * that can be parsed without a lookahead token. That makes it
+ * feasible for an optional argument that may need to be skipped:
+ * skipping can only be accomplished by pushing back the token into
+ * the lexer, and that only works when there is no lookahead token.
+ *
+ * Sequences of 0 or more optional arguments are scanned using either
+ * function_arglist_backup or function_arglist_nonbackup. The first
+ * is used when optional arguments are followed by at least one
+ * mandatory argument: in that case optional arguments may be skipped
+ * by either a false predicate (in which case the expression will be
+ * pushed back as one or more tokens, preceded by a BACKUP token) or
+ * by using \default.
+ *
+ * If optional arguments are at the end of the argument list, they are
+ * instead scanned using function_arglist_nonbackup: here the only
+ * manner to enter into skipping of optional arguments is the use of
+ * \default.
+ *
+ * The argument list of a normal function call is parsed using
+ * function_arglist. The part of an argument list before a mandatory
+ * argument is parsed using function_arglist_optional.
+ *
+ * The difference is that leading optional arguments are scanned using
+ * function_arglist_nonbackup and function_arglist_backup,
+ * respectively.
+ *
+ * Most other details are obvious in the rules themselves.