reserve n for Element of NAT,
  a, b for Real;

theorem Th9:
  a <= b implies Closed-Interval-TSpace(a,b) is interval
proof
  set X = Closed-Interval-TSpace(a,b);
  assume a <= b;
  then [.a,b.] = [#]X by TOPMETR:18;
  hence [#]X is interval;
end;
