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

theorem Th28: :: MEASURE5:12,15
  p <= q implies ].q,p.[ = {}
proof
  assume p <= q;
  then [.q,p.[ = {} by Th27;
  hence thesis by Th22,XBOOLE_1:3;
end;
