reserve x for set,
  p,q,r,s,t,u for ExtReal,
  g for Real,
  a for Element of ExtREAL;

theorem Th20: :: MEASURE5:13
  ].r,r.[ = {}
proof
  let p;
  thus p in ].r,r.[ implies p in {}
  proof
    assume p in ].r,r.[;
    then ex a st p = a & r < a & a < r;
    hence thesis;
  end;
  thus thesis;
end;
