reserve n,m,k for Nat,
  X,Y,Z for set,
  f for Function of X,Y,
  H for Subset of X;

theorem Th4:
  X is infinite & Y is finite implies X \ Y is infinite
proof
  X \/ Y = (X \ Y) \/ Y by XBOOLE_1:39;
  hence thesis by Th3;
end;
