theorem Th17:
  Int K /\ Int L = Int(K /\ L)
proof
  Int K /\ Int L = (Cl K` \/ Cl L`)` by XBOOLE_1:53
    .= (Cl(K` \/ L`))` by PRE_TOPC:20
    .= (Cl (K /\ L)`)` by XBOOLE_1:54;
  hence thesis;
end;
