unique_drinkers

# two folks have different preferences if one likes a beer the other doesn't.
differ(a, b) :- person(b), likes(beer, a), !likes(beer, b).
differ(a, b) :- person(a), !likes(beer, a), likes(beer, b).

# what you want to write
unique(a) :- ∀b. (a = b or differ(a,b)).

# how you have to write it.
unique(a) :- not exists_same(a).
exists_same(a) :- person(a), person(b), !differ(a,b), a != b.