reserve a,b,c,d for Ordinal;
reserve l for non empty limit_ordinal Ordinal;
reserve u for Element of l;
reserve A for non empty Ordinal;
reserve e for Element of A;
reserve X,Y,x,y,z for set;
reserve n,m for Nat;

theorem Th21:
  numbering X is_isomorphism_of RelIncl ord-type X, RelIncl On X
  proof
    set R1 = RelIncl ord-type X;
    set R2 = RelIncl On X;
    R2,R1 are_isomorphic by WELLORD2:def 2; then
    R1,R2 are_isomorphic by WELLORD1:40;
    hence thesis by WELLORD1:def 9;
  end;
