motivation for a pro subject comes from the grammatical of sentences such as (1) and (2), where the subject of the infinitival "to"-clause, though not overtly expressed, is understood to be controlled by an argument of the main clause. in (1a), the subject of "control" is understood to be the same person that issued the promise, namely "john". this is annotated in (1b) by co-indexing "john" with pro, which indicates that the pro subject of [ "to control the situation"] co-refers with "john". in (2a), the subject of "sleep" is understood to be the same person that was convinced, namely "bill". this is annotated in(2b) by co-indexing "bill" with pro, which indicates that the pro subject of [ "to sleep"] co-refers with "bill".