x_{1} (set) is the unique region/part in the Venn diagram of
sets x_{2} (set of sets; exhaustive) such that each of its (i.e.: x_{1}'s) members is a member of exactly each of the explicitly-mentioned elements of x_{3} (set of sets; subset of x_{2}; exhaustive) and of no other elements of x_{2}.

The Venn diagram of exactly two sets A and B has exactly four regions/parts which would qualify for valid submissions to x_{1} (when x_{2} = Set(A,B)), namely: A-B, B-A, A-intersect-B, and the complement of A-union-B; for the first two options, see "kei'i"; under the previous assumptions, these would respectively have x_{3 }= A, B, Set(A,B), and the empty set (again, RESPECTIVELY).