reserve TS for 1-sorted,
  K, Q for Subset of TS;
reserve TS for TopSpace,
  GX for TopStruct,
  x for set,
  P, Q for Subset of TS,
  K , L for Subset of TS,
  R, S for Subset of GX,
  T, W for Subset of GX;

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;
