Here are the data structures with brief descriptions:

Cly_ctx	Libyang context handler
Cly_err_item	Libyang full error structure
Cly_set	Structure to hold a set of (not necessary somehow connected) ::lyd_node or ::lys_node objects. Caller is supposed to not mix the type of objects added to the set and according to its knowledge about the set content, it is supposed to access the set via the sset, dset or set members of the structure
Cly_set_set	Set array of ly_set It is kept in union to keep ly_set generic for data as well as schema trees