theorem
  for s1,t1,s2,t2 for P,P1 being Subset of TOP-REAL 2 st s1<s2 & t1<t2 &
  P = { p where p is Point of TOP-REAL 2: p`1 = s1 & p`2 <= t2 & p`2 >= t1 or
  p`1 <= s2 & p`1 >= s1 & p`2 = t2 or p`1 <= s2 & p`1 >= s1 & p`2 = t1 or
  p`1 = s2 & p`2 <= t2 & p`2 >= t1} &
  P1 = {pa where pa is Point of TOP-REAL 2:
  s1<pa`1 & pa`1<s2 & t1 <pa`2 & pa`2 < t2} holds Cl P1 = P \/ P1 by Lm10;
