reserve A,B for Ordinal,
  K,M,N for Cardinal,
  x,x1,x2,y,y1,y2,z,u for object,X,Y,Z,X1,X2, Y1,Y2 for set,
  f,g for Function;
reserve m,n for Nat;
reserve x1,x2,x3,x4,x5,x6,x7,x8 for object;
reserve A,B,C for Ordinal,
  K,L,M,N for Cardinal,
  x,y,y1,y2,z,u for object,X,Y,Z,Z1,Z2 for set,
  n for Nat,
  f,f1,g,h for Function,
  Q,R for Relation;
reserve n,k for Nat;

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;
