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

theorem :: MEASURE6:36
  ].p,-infty.[ = {}
proof
  not ex x being object st x in ].p,-infty.[
  proof
    given x being object such that
A1: x in ].p,-infty.[;
    reconsider s = x as ExtReal by A1;
    s < -infty by A1,Th4;
    hence contradiction by XXREAL_0:5;
  end;
  hence thesis;
end;
