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
  S is open & S c= T implies S c= Int T
proof
  assume that
A1: S is open and
A2: S c= T;
  Int S = S by A1,Th23;
  hence thesis by A2,Th19;
end;
