reserve
  a,b,c,d,e for Ordinal,
  m,n for Nat,
  f for Ordinal-Sequence,
  x for object;
reserve S,S1,S2 for Sequence;

theorem Th18:
  a |^|^ 2 = exp(a, a)
  proof
    2 = succ 1;
    hence a |^|^ 2 = exp(a, a|^|^1) by Th14 .= exp(a, a) by Th16;
  end;
