An integerliteral is a sequence of digits representing an integer constant.
整型字面值是一个代表整型常量的数字序列。
2
An integerliteral, followed by a plus sign, followed by another integerliteral.
一个整数文字,后面跟一个加号,再跟另一个整数文字。
3
The second production describes in regex-like fashion more specifically what an integerliteral looks like: a contiguous sequence of one or more numeric digits.