theorem
  F is open implies F|M is open
proof
  assume
A1: F is open;
  let Q be Subset of T|M;
  assume Q in F|M;
  then consider R being Subset of T such that
A2: R in F and
A3: R /\ M = Q by Def3;
  reconsider R as Subset of T;
A4: Q = R /\ [#](T|M) by A3,PRE_TOPC:def 5;
  R is open by A1,A2;
  hence thesis by A4,Th24;
end;
