string_list : string_list COMMA STRING
                       | STRING
                       | empty

def p_string_list(self, t):
        """string_list : string_list COMMA STRING
                       | STRING
                       | empty"""

        if len(t) == 4:
            t[0] = t[1]
            t[0].append(t[3])
        elif t[1]:
            t[0] = [t[1]]
        else:
            t[0] = []