def control(self):
        """
        control    : 'if' ctrl_exp block ('elif' ctrl_exp block)* ('else' block)
        """
        self.eat(TokenTypes.IF)
        ctrl = self.expression()
        block = self.block()
        ifs = [If(ctrl, block)]
        else_block = Block()
        while self.cur_token.type == TokenTypes.ELIF:
            self.eat(TokenTypes.ELIF)
            ctrl = self.expression()
            block = self.block()
            ifs.append(If(ctrl, block))
        if self.cur_token.type == TokenTypes.ELSE:
            self.eat(TokenTypes.ELSE)
            else_block = self.block()
        return ControlBlock(ifs, else_block)
control    : 'if' ctrl_exp block ('elif' ctrl_exp block)* ('else' block)