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

theorem
  r <= s implies [.r,s.] \ {r,s} = ].r,s.[
proof
  assume r <= s;
  then
A1: [.r,s.] = ].r,s.[ \/ {r,s} by Th128;
A2: not r in ].r,s.[ by Th4;
  not s in ].r,s.[ by Th4;
  hence thesis by A1,A2,ZFMISC_1:121;
end;
