14.3. DSL Example

[when] indicates the scope of the expression (that is, whether it is valid for the LHS or the RHS of a rule).

The part after the bracketed keyword is the expression that you use in the rule.

The part to the right of the equal sign ("=") is the mapping of the expression into the rule language. The form of this string depends on its destination, RHS or LHS. If it is for the LHS, then it ought to be a term according to the regular LHS syntax; if it is for the RHS then it might be a Java statement.