theorem Th50:
  {} is_CRS_of m iff m = 0
proof
  set fp=<*>INT;
  thus {} is_CRS_of m implies m = 0 by Th49,CARD_1:27;
  assume m = 0;
  then
A1: len fp = m;
  {} = rng fp & for b be Nat st b in dom fp holds fp.b in Class(Cong(m),b -'1);
  hence thesis by A1;
end;
