theorem
  not M is finite implies not M+`N is finite & not N+`M is finite
proof
  assume
A1: not M is finite;
  M c= N or N c= M;
  then M c= N & not N is finite or M+`N = M & N+`M = M by A1,Th75;
  hence thesis by A1,Th75;
end;
