docs: public int depth() {
          if (isLeaf()) {
              return 0;
          }
          int maxDepth = 0;
          List<Tree> kids = children();
          for (Tree kid : kids) {
              int curDepth = kid.depth();
              if (curDepth > maxDepth) {
                  maxDepth = curDepth;
              }
          }
          return maxDepth + 1;
      }