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 f being Real_Sequence, N,M being Element of NAT holds Sum(f,N,M) +
  f.(N+1) = Sum(f,N+1,M) by Lm15;
