
theorem
  for T1,T2 being TopStruct, T being Refinement of T1,T2
  holds T is Refinement of T2,T1
proof
  let T1, T2 be TopStruct, T be Refinement of T1,T2;
A1: the carrier of T = (the carrier of T1) \/ (the carrier of T2) by Def6;
  (the topology of T1) \/ (the topology of T2) is prebasis of T by Def6;
  hence thesis by A1,Def6;
end;
