docs: function (subpath, preserve, newValue) {
      var args = Util.resolveArgs(
        arguments,
        function (x) { return Util.canRepresentSubpath(x) ? 'subpath' : null; },
        '?preserve',
        'newValue'
      );
      update(this, args.subpath, merge.bind(null, args.preserve, args.newValue));
      return this;
    }