// queue:    inserts and removes happen at opposite ends
// insfirst: elements flow from "first," "head" or "front", to "last", "tail" or "back"
// inselem:  inserts happen via an element-level operation and removes happen via the api-provided "last"/"tail"/"back"

#define BOP_INIT(lst, ui, iters, i) BFX_INSERT_FIRST(B_UserItem, lst, ui[i])

#define BOP_INSERT(lst, ui, iters, i) BFX_INSERT_BEFORE(B_UserItem, lst, ui[i], iters[i-1])
#define BOP_REMOVE(lst, ui, iters, i) BFX_REMOVE_LAST(B_UserItem, lst)