Complex sentence: In a predicate a clause of the form x\\ in\\ is either true or false, or when used as a constraint means that x can only be one of the values 1,2, or 3.
Split into simpler sentences: In a predicate a clause of the form x\\ in\\ is either true or false. When such a clause is used as a constraint it means that x can only be one of the values 1,2, or 3.