reserve T for TopStruct;
reserve GX for TopSpace;

theorem Th3:
  for T being 1-sorted, P being Subset of T holds [#]T \ ([#]T \ P ) = P
proof
  let T be 1-sorted, P be Subset of T;
  [#]T \ ([#]T \ P) = P /\ [#]T by XBOOLE_1:48;
  hence thesis by XBOOLE_1:28;
end;
