// stack:    inserts and removes happen at the same end
// inslast:  stack's action happens at the end called "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_LAST(B_UserItem, lst, ui[i])

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