reserve T for TopSpace;

theorem Th17:
  for F being Subset-Family of T holds Int F is open
proof
  let F be Subset-Family of T;
  now
    let A be Subset of T;
    assume A in Int F;
    then ex B being Subset of T st A = Int B & B in F by Def1;
    hence A is open;
  end;
  hence thesis by TOPS_2:def 1;
end;
