theorem Th27:
  for s1,t1,s2,t2 for P being Subset of TOP-REAL 2 st
  P = { p0 where p0 is Point of TOP-REAL 2:s1<p0`1 & p0`1<s2
  & t1<p0`2 & p0`2<t2} holds P is open
proof
  let s1,t1,s2,t2;
  let P be Subset of TOP-REAL 2;
  assume P = { p0 where p0 is Point of TOP-REAL 2:s1<p0`1 & p0`1<s2
  & t1<p0`2 & p0`2<t2};
  then P={|[sa,ta]|:s1<sa & sa<s2 & t1<ta & ta<t2} by Th21;
  hence thesis by Th18;
end;
