21.  For the most difficult cases, there are not enough spots.

22.  Frank Jr. also knows he has a difficult case to make.

23.  He was a discoverer of drugs used to treat difficult cases of tuberculosis.

24.  His experiment has been so successful that the sponsoring agency now wants to take on even more difficult cases for deinstitutionalization.

25.  His former students, now practicing around the world, refer difficult cases to him.

26.  In a move reserved for extremely difficult cases, authorities turned to Frank Bender, a forensic artist from Philadelphia.

27.  In many cases, the pediatricians are expected to serve as an early-warning system, referring the most difficult cases to counselors and other specialists.

28.  In the past, Ms. Reno has named senior prosecutors to handle difficult cases.

29.  It was a very difficult case.

30.  It is a difficult case, one that may yet hinge on a Supreme Court refinement of what is meant by an imminent threat.