IsExpression ::= Expression IS Identifier
The IS operator can be used to test the class membership of a non-terminal value.
For instance,
AnT IS Statement
will yield TRUE if AnT denotes a non-terminal value, of the
Statement class or one of its derived-classes. It will return
FALSE if AnT denotes a non-terminal value of another class;
and will abort the script with an ad hoc error message if it
does not denote a non-terminal value.
The SYS environment
provides primitives that can be used to check whether a given value is a non-terminal or not.