string_list : string_list COMMA IDENT
                       | IDENT
                       | empty

def p_string_list(self, p):
        '''string_list : string_list COMMA IDENT
                       | IDENT
                       | empty'''
        if p[1] is None:
            p[0] = []
        elif len(p) == 4:
            p[1].append(p[3])
            p[0] = p[1]
        elif len(p) == 2:
            p[0] = [p[1]]