reserve U for Universe;
reserve x for Element of U;
reserve U1,U2 for Universe;

theorem
  for C,D being CategorySet holds
  Obj Functors(C,D) = Funct(SetToCat C,SetToCat D) &
  Mor Functors(C,D) = NatTrans(SetToCat C,SetToCat D) by NATTRA_1:def 17;
