code: function throwIf (target /*, illegal... */) {
      Array.prototype.slice.call(arguments, 1).forEach(function (name) {
        if (options[name]) {
          throw new Error('Cannot set ' + name + ' and ' + target + 'together');
        }
      });
    }
docs: Helper function which throws an `Error` in the event that any of the rest of the arguments is present in `options`.