/* Characterizes a link as a child pointer or a thread. */
enum tavl_tag
  {
    TAVL_CHILD,                     /* Child pointer. */
    TAVL_THREAD                     /* Thread. */
  };

/* An TAVL tree node. */
struct tavl_node
  {
    struct tavl_node *tavl_link[2]; /* Subtrees. */
    void *tavl_data;                /* Pointer to data. */
    unsigned char tavl_tag[2];      /* Tag fields. */
    signed char tavl_balance;       /* Balance factor. */
  };