loop       : 'for' init; ctrl; inc block

def loop(self):
        """
        loop       : 'for' init; ctrl; inc block
        """
        self.eat(TokenTypes.FOR_LOOP)
        init = NoOp()
        if self.cur_token.type != TokenTypes.SEMI_COLON:
            init = self.assign_statement()
        else:
            self.eat(TokenTypes.SEMI_COLON)

        ctrl = NoOp()
        if self.cur_token.type != TokenTypes.SEMI_COLON:
            ctrl = self.expression()
        self.eat(TokenTypes.SEMI_COLON)

        inc = NoOp()
        if self.cur_token.type != TokenTypes.LBRACE:
            inc = self.assign_statement()

        block = self.block()
        return ForLoop(init, ctrl, inc, block)