void insertRedBlack(Tree t, Item it)
{
   t->root = insertRB(t->root, it, 0);
   t->root->colour = BLACK;
}
Link insertRB(Link t, Item it, int inRight)
{
   if (t == NULL) return newNode(it,RED);
   if (red(L(t)) && red(R(t))) {
      // split 4-node and promote middle value
      // performed as we descend tree
   }
   // recursive insert cases (cf. regular bst)
   // then re-arrange links/colours after insert
   return t';
}