code: public final int prev(int node) {
          final int left = left(node);
          if (left != NIL) {
              return last(left);
          } else {
              int parent = parent(node);
              while (parent != NIL && node == left(parent)) {
                  node = parent;
                  parent = parent(parent);
              }
              return parent;
          }
      }
docs: Return the highest node that is strictly less than <code>node</code>.