code: 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;
      }
docs: Finds the channels of the tree.  The channels is defined as the length
  of the longest path from this node to a leaf node.  Leaf nodes
  have channels zero.  POS tags have channels 1. Phrasal nodes have
  channels &gt;= 2.
  
  @return the channels