theorem Th20:
  Int T \/ Int W c= Int(T \/ W)
proof
A1: Cl(T` /\ W`) c= Cl T` /\ Cl W` by PRE_TOPC:21;
  Int T \/ Int W = ((Cl T`) /\ Cl W`)` by XBOOLE_1:54;
  then Int T \/ Int W c= (Cl(T` /\ W`))` by A1,SUBSET_1:12;
  hence thesis by XBOOLE_1:53;
end;
