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 Th19:
  for X being ordinal-membered set holds rng numbering X = X
  proof
    let X be ordinal-membered set;
    thus rng numbering X = On X by Th18 .= X by Th2;
  end;
