theorem Th23: :: JORDAN1:29

for s1, s2, t1, t2 being Real 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)