theorem Th5:
  R,RelIncl A are_isomorphic & R,RelIncl B are_isomorphic implies A = B
proof
  assume that
A1: R,RelIncl A are_isomorphic and
A2: R,RelIncl B are_isomorphic;
  RelIncl A,R are_isomorphic by A1,WELLORD1:40;
  hence thesis by A2,Th4,WELLORD1:42;
end;
