link_t* list_prepend(link_t* head, int value);
link_t* list_insert(link_t* head, unsigned index, int value);
link_t* list_delete(link_t* head, unsigned index);
link_t* list_prepend(link_t* head, int value);
link_t* list_insert(link_t* head, unsigned index, int value);
link_t* list_delete(link_t* head, unsigned index);