The %if construct doesn't recognize "_" as a legas first character in a
macro name.  For example
	%define _foo bar
	...
	%if _foo == "bar"
	...
gets an error, wheras
	%define foo bar
	...
	%if foo == "bar"
	...
does not.

Suggest changing line 265 in expression.c
	    } else if (isalpha(*p)) {
to
	    } else if (isalpha(*p) || *p == '_') {