/* Characterizes a link as a child pointer or a thread. */
enum rtbst_tag
  {
    RTBST_CHILD,                     /* Child pointer. */
    RTBST_THREAD                     /* Thread. */
  };

/* A threaded binary search tree node. */
struct rtbst_node
  {
    struct rtbst_node *rtbst_link[2]; /* Subtrees. */
    void *rtbst_data;                 /* Pointer to data. */
    unsigned char rtbst_rtag;         /* Tag field. */
  };