code: function (subpath, f) {
      var args = Util.resolveArgs(arguments, '?subpath', 'f');
      update(this, args.subpath, args.f);
      return this;
    }
docs: Update binding value.
  @param {String|Array} [subpath] subpath as a dot-separated string or an array of strings and numbers
  @param {Function} f update function
  @return {Binding} this binding