+/* Parses one function argument from the N_INPUT tokens in INPUT
+ Each argument to a macro function is one of:
+
+ - A quoted string or other single literal token.
+
+ - An argument to the macro being expanded, e.g. !1 or a named argument.
+
+ - !*.
+
+ - A function invocation.
+
+ Each function invocation yields a character sequence to be turned into a
+ sequence of tokens. The case where that character sequence is a single
+ quoted string is an important special case.
+*/