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

theorem Th402:
  r < s implies ].r,s.] \ ].r,s.[ = {s}
proof
  [.s,s.] = {s} by Th17;
  hence thesis by Th188;
end;
