/* Color of a red-black node. */
enum trb_color
  {
    TRB_BLACK,                     /* Black. */
    TRB_RED                        /* Red. */
  };

/* Characterizes a link as a child pointer or a thread. */
enum trb_tag
  {
    TRB_CHILD,                     /* Child pointer. */
    TRB_THREAD                     /* Thread. */
  };

/* An TRB tree node. */
struct trb_node
  {
    struct trb_node *trb_link[2];  /* Subtrees. */
    void *trb_data;                /* Pointer to data. */
    unsigned char trb_color;       /* Color. */
    unsigned char trb_tag[2];      /* Tag fields. */
  };