
theorem Th20:
  for T being non empty TopSpace, G,F being Subset of T st G is
  open & F is closed holds F \ G is closed
proof
  let T be non empty TopSpace, G,F be Subset of T such that
A1: G is open & F is closed;
  F \ G = F /\ G` by SUBSET_1:13;
  hence thesis by A1;
end;
