
theorem Th21:
  for S,T being non empty RelStr, g being Function of S,T, d being
  Function of T,S st g = g*d*g holds g*d is idempotent
proof
  let S,T be non empty RelStr, g be Function of S,T, d be Function of T,S;
  assume g = g*d*g;
  hence (g*d)*(g*d) = g*d by RELAT_1:36;
end;
