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A trigger is prescribed by an interface to be handled by an
implementation as is the condition under which it occurs. Collectively
this is referred to as a declarative statement. The condition is
expressed as a guard, the trigger in an on. The code that
is executed when both the guard expression evaluates to true and
the trigger occurs, is called the imperative statement
(See Imperative Statements).
declarative-statement ::= guard | on | "{" declarative-statement "}"
The combination of all declarative statements leading up to a
imperative statement is also called the declarative
prefix. The combination of all guards, the on and
the imperative statement is sometimes referred to as a
transition.
transition ::= prefix imperative-statement prefix ::= <guards> <on>