Availability:C-language interface function
void* PL_set_query_data(qid_t
qid, unsigned int offset, void* data)
void* PL_query_data(qid_t
qid, unsigned int offset)Associate user data with a query. offset must be smaller than
PL_MAX_QUERY_DATA
(currently 2). PL_set_query_data()
returns the old value.