/* Characterizes a link as a child pointer or a thread. */
enum tbst_tag
  {
    TBST_CHILD,                     /* Child pointer. */
    TBST_THREAD                     /* Thread. */
  };

/* A threaded binary search tree node. */
struct tbst_node
  {
    struct tbst_node *tbst_link[2]; /* Subtrees. */
    void *tbst_data;                /* Pointer to data. */
    unsigned char tbst_tag[2];      /* Tag fields. */
  };

