The relationship between two sets, A and B, is a trilemma if there are three possibilities. There are phenomena that are members of (1) both set A and set B, (2) neither set A, nor set B, or only set A, but not set B. There are no phenomena that are members of set B that are not also members of set A. In other words, all elements of set A are also members of set B, but not all elements of set B are elements of set A.
J Hopkins: three possibilities; three possibilities/permutations.