:- use_module(library(qwerty)).

:- (type T ---> a ; b), % T has two constructors, a and b.
   (type _ ---> p(T)). % p/1 demands an argument of type T, i.e. a or b.
:- type _ ---> X = X. % (=)/2 demands arguments of the same type.

p(a).
p(b).
p(X) :- X = c. % FAIL, c is not in T.