BT_POP
while ((parent = BT_POP(btstack)) != NULL) {
parent = BT_POP(btstack);
while ((parent = BT_POP(btstack)) != NULL) {
if ((parent = BT_POP(&btstack)) == NULL)
if ((parent = BT_POP(&btstack)) == NULL)
while ((parent = BT_POP(btstack)) != NULL) {