Yes, that's also possible with this definition! For example: Given Ls0 = [a,b,c] and Ls = [a,c], which element (if any) was dropped:

    ?- drop(X, [a,b,c], [a,c]).
    X = b ;
    false.

As another example, if both Ls0 and Ls are [a,b,c]:

    ?- drop(X, [a,b,c], [a,b,c]).
    dif(X, c),
    dif(X, b),
    dif(X, a).

This means that X must be different from each of these elements.

More generally, which elements can be dropped at all from the list [a,b,c]:

    ?- drop(X, [a,b,c], _).
    X = a ;
    X = b ;
    X = c ;
    dif(X, c),
    dif(X, b),
    dif(X, a).

Interestingly, the predicate definition does not even use "=" in its clauses. In Prolog, (=)/2 is a built-in predicate that means unification. In the definition above, we use implicit unification instead of explicit unification.