VISIT
void twalk(const void *, void (*)(const void *, VISIT, int));
twalk(const void *vroot, void (*action)(const void *, VISIT, int))
if (root != (node *)0 && action != (void (*)(const void *, VISIT, int))0)
trecurse(node *root, void (*action)(const void *, VISIT, int), int level)
visit_nodes(const void *nodep, VISIT which, int depth)