in (17a) above, the subject of the verb in the main clause (john promised mary) is john, so pro is interpreted as referring to john, while in (17b) to sleep is an action performed by bill, and pro is interpreted as referring to bill. and in (17c), pro is not controlled by any antecedent, and so can be paraphrased as 'for someone to love her is for someone to know her'; this is called impersonal pro or arbitrary pro.