Parts of an EBNF production enclosed by "[" and "]" are optional. They may appear exactly once in the language, or they may be skipped. The minus sign in the last production above is optional, both -7 and 7 are allowed.
The repetition is another useful construct. Any part of a production enclosed by "{" and "}" may appear any number of times in the language described (including zero, that is, it may also be skipped). ArgList above is an example: an argument list is a single expression or a list of any number of expressions separated by commas.