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

theorem
  r < s implies [.s,p.[ c= ].r,p.[
proof
  assume
A1: r < s;
  let t;
  assume
A2: t in [.s,p.[;
  then s <= t by Th3;
  then
A3: r < t by A1,XXREAL_0:2;
  t < p by A2,Th3;
  hence thesis by A3,Th4;
end;
