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.] \ {s} = [.r,s.[
proof
  assume r <= s;
  then
A1: [.r,s.] = [.r,s.[ \/ {s} by Th129;
  not s in [.r,s.[ by Th3;
  hence thesis by A1,ZFMISC_1:117;
end;
