theorem
  a <= b implies ClosedHyperInterval(a,b) is non empty
  proof
    assume a <= b; then
    ClosedHyperInterval(a,a) c= ClosedHyperInterval(a,b) by Th49;
    then {a} c= ClosedHyperInterval(a,b) by Th48;
    hence thesis;
  end;
