opt_order :
                     | ORDER LPAREN IDENTIFIER RPAREN

def p_opt_order(self, p):
        '''opt_order :
                     | ORDER LPAREN IDENTIFIER RPAREN'''
        if len(p) > 1:
            if p[3] not in 'CF':
                raise PythranSyntaxError("Invalid Pythran spec. "
                                         "Unknown order '{}'".format(p[3]))
            p[0] = p[3]
        else:
            p[0] = None