reserve Y for non empty set,
  G for Subset of PARTITIONS(Y),
  A,B,C,D,E,F for a_partition of Y;

theorem
  for B,C,D,b,c,d being object, h being Function
    st h = (B,C,D) --> (b,c,d)
  holds rng h = {h.B,h.C,h.D} by Lm2;
