21.   Simple in concept but complex in implementation, gene therapy may yet prove more difficult to master than researchers now think.

22.   Some investors are less concerned German debt sales will prove more difficult in the coming years.

23.   Still, dividing the burden of emission reductions proved difficult, and the threat of lawsuits hung over the negotiations.

24.   The match against Courville should prove more difficult.

25.   Those extraditions may prove more difficult.

26.   Unlike northern Afghanistan, where the Taliban forces retreated in days, the south may prove more difficult.

27.   And the rules are likely to prove difficult to enforce, given the ease with which many migrants find jobs and settle into the city.

28.   But finding a successor so he could retire proved difficult.

29.   But since fighting began five months ago, gains by government forces have proven difficult to maintain and it remained possible the rebel Kosovo Liberation Army could still regroup.

30.   Economic coordination might prove more difficult.