skip duplicate type assertions

[mndrix]

A predicate declared like this

%%  foo(A:atom, B:atom)
foo(X,X) :-
  true.

is expanded into

foo(X,X) :-
  the(atom, X),
  the(atom, X),
  true.

Those two type assertions are identical and should be not be repeated. This can be fixed by just calling sort(Assertions, Sorted) which removes duplicates from Assertions.