ex_expression : OP_EXISTS cmp_expression
                         | cmp_expression

def p_ex_expression(tok):
        """ex_expression : OP_EXISTS cmp_expression
                         | cmp_expression"""
        if len(tok) == 3:
            tok[0] = UnaryOperationRule(tok[1], tok[2])
        else:
            tok[0] = tok[1]