reserve A, B for non empty preBoolean set,
  x, y for Element of [:A,B:];
reserve X for set,
  a,b,c for Element of [:A,B:];
reserve a for Element of [:Fin X, Fin X:];
reserve A for set;

theorem Th18:
  FinPairUnion A is having_a_unity
proof
  [{}.A, {}.A] is_a_unity_wrt FinPairUnion A by Th17;
  hence thesis by SETWISEO:def 2;
end;
