14 Expressions

The syntax in this section contains the following superscript and subscript symbols:

The opt subscript is attached to symbols that are optional.
The allowIn superscript is attached to nonterminals that can be defined with a production rule that contains the in operator.
The noIn superscript is attached to nonterminals with definitions that do not include a production rule that contains the in operator. This superscript is necessary to avoid conflicts between the in operator as part of a relational expression and the in operator as part of a for statement.
The β superscript is shorthand notation that denotes a nonterminal with a pair of definitions: an allowIn version and a noIn version.

Current page: http://livedocs.adobe.com/specs/actionscript/3/as3_specification120.html