code: function(tree, n, m) {
      return tree[n].fc < tree[m].fc ||
        (tree[n].fc == tree[m].fc && zip_depth[n] <= zip_depth[m]);
  }
docs: /* ==========================================================================
  Compares to subtrees, using the tree depth as tie breaker when
  the subtrees have equal frequency. This minimizes the worst case length.