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

theorem Th403:
  r < t implies [.r,t.] \ ].r,t.] = {r}
proof
  [.r,r.] = {r} by Th17;
  hence thesis by Th192;
end;
