code: function(a, b)
  {
      return Math.abs(a.bb_l + a.bb_r - b.bb_l - b.bb_r) + Math.abs(a.bb_b + a.bb_t - b.bb_b - b.bb_t);
  }
docs: **** Subtree Functions Would it be better to make these functions instance methods on Node and Leaf?