theorem Th46:
  inside_of_rectangle(a,b,c,d) c= closed_inside_of_rectangle(a,b,c,d)
proof
  let x be object;
  assume x in inside_of_rectangle(a,b,c,d);
  then ex p st x = p & a < p`1 & p`1 < b & c < p`2 & p`2 < d;
  hence thesis;
end;
