10.4.4.6 illegal

A trigger can be explicitly marked as being illegal in a certain state. In that case, illegal must be the only imperative statement for that trigger.

illegal ::= "illegal" ";"

For example:

on hello: illegal;

Note: Since 2.14.0, a declarative-statement followed by an illegal can be completely omitted, since it has the same meaning. It is however still available for backwards compatibility.