theorem
  M in N or M c= N implies K = 0 or
  exp(K,M) c= exp(K,N) & exp(M,K) c= exp(N,K)
proof
  assume that
A1: M in N or M c= N and
A2: K <> 0;
  thus exp(K,M) c= exp(K,N) by A1,A2,Th91;
  M = 0 implies exp(M,K) = 0 by A2;
  hence thesis by A1,Th91;
end;
