reserve a,b for object, I,J for set;

theorem Lem3:
  for m,n being bag of I holds m-'n divides m
  proof
    let m,n be bag of I;
    let a;
    m.a-'n.a <= m.a by NAT_D:35;
    hence (m-'n).a <= m.a by PRE_POLY:def 6;
  end;
