docs: function toArray(tree, f) {
  	var out = [];
  	if (tree.root) {
  		_util.rootToArray(tree.root, f, out);
  	}
  	var tail = tree.tail;
  	for(var i = 0, l = tail.length; i<l; i++) {
  		out.push(f(tail[i]));
  	}
  	return out;
  }