theorem Th51:
  for A,B be Subset of RLS st A c= B & B is Affine holds Affin A c= B
 proof
  let A,B be Subset of RLS;
  assume A c=B & B is Affine;
  then B in {C where C is Affine Subset of RLS:A c=C};
  hence thesis by SETFAM_1:3;
 end;
