theorem Th23:
  for s1,s2,t1,t2 holds
  { p0 where p0 is Point of TOP-REAL 2:s1<p0`1 & p0`1<s2 & t1<p0`2 & p0`2<t2}
  is Subset of TOP-REAL 2
proof
  let s1,s2,t1,t2;
  { |[ sc,tc ]|: s1<sc & sc<s2 & t1<tc & tc<t2}
  is Subset of TOP-REAL 2 by Lm2,Lm7;
  hence thesis by Th21;
end;
