Chapter 11 Subset check fix

[nicoabie]

Hi,
The solution provided will asset that subset([a,b,c,a], [a, b, c]). is true.
It is missing checking that the first parameter is a set itself.
Here I propose a fix using the set definition implemented in a previous chapter.

set([], []).
set([H|T], X):- member(H, T), !, set(T, X).
set([H|T], [H|X]):- set(T, X).

subset_check(SS, S):-
  set(SS, SS),
  all_members(SS, S).

all_members([], _).
all_members([H|T], S):-
  member(H, S),
  !,
  all_members(T, S)

Thanks a lot for making this repository.