reserve c, c1, c2, d, d1, d2, e, y for Real,
  k, n, m, N, n1, N0, N1, N2, N3, M for Element of NAT,
  x for set;

theorem
  for n,k st k >= 1 & n! <= k & k < (n+1)! holds Step1(k) = n! by Def6;
