theorem Th28:
  for s1,t1,s2,t2 for P being Subset of TOP-REAL 2 st
  P = { pq where pq is Point of TOP-REAL 2:
  not (s1<=pq`1 & pq`1<=s2 & t1<=pq`2 & pq`2<=t2)} holds P is open
proof
  let s1,t1,s2,t2;
  let P be Subset of TOP-REAL 2;
  assume P= { pq where pq is Point of TOP-REAL 2:
  not (s1<=pq`1 & pq`1<=s2 & t1<=pq`2 & pq`2<=t2)};
  then P = {|[sb,tb]| : not (s1<=sb & sb<=s2 & t1<=tb & tb<=t2)} by Th22;
  hence thesis by Th19;
end;
