This function sets the element set name for retrieval.
Call this function before yaz_search or yaz_present to specify the element set name for records to be retrieved.
If this function appears to have no effect, see the description of the piggybacking option in yaz_connect.
The connection resource returned by yaz_connect.
Most servers support F (for full records) and B (for brief records).
Returns true on success or false on failure.