VISIT
void (*action)(const void *, VISIT, int))
trecurse(const node_t *root, void (*action)(const void *, VISIT, int),
void twalk(const posix_tnode *, void (*)(const posix_tnode *, VISIT, int));
typedef void (*cmp_fn_t)(const posix_tnode *, VISIT, int);
treewalk(const posix_tnode *node, VISIT v, int level)