21.   It may turn out that even a disciplined system will prove increasingly expensive.

22.   Officials hope the system will prove so beneficial that shippers will finance expansion and future operations.

23.   On Monday, the advancing weather system proved more powerful than originally forecast.

24.   Other biological cleaning systems have proved useful.

25.   Preliminary systems have proved capable of locking onto a few geometric features and tracking them as the wheelchair moves.

26.   Seeking to convey their excitement, they produced a body of work whose originality was so extraordinary that the Soviet system proved unable to tolerate it.

27.   So far, the system has proven to be an asset not only in firefighting decisions, but in the other emergencies to which Forest Service personnel respond.

28.   Specially designed business systems have proven capable of reliably handling large volumes of complex transactions in real time, he notes.

29.   Still, the company says its data system will eventually prove successful.

30.   The ceiling soffits had rotted, the terrazzo floors had been damaged by carpet glue, and the mechanical systems proved inadequate.