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

theorem Th18: :: MEASURE5:13
  [.r,r.[ = {}
proof
  let p;
  not ex p st p in [.r,r.[
  proof
    given p such that
A1: p in [.r,r.[;
    ex a st p = a & r <= a & a < r by A1;
    hence thesis;
  end;
  hence thesis;
end;
