code: function(callable, root, isMaster) {
    rfr = callable.bind(callable);
  
    /**
     * A read-only property tells whether this rfr instance is a master one.
     * Call `require('rfr')` to get a master rfr instance.
     * User created rfr instances, such as `require('rfr')({ root: '...' })` are
     * not master ones.
     * @type {Boolean}
     */
    Object.defineProperty(rfr, 'isMaster', {
      configurable: false,
      enumerable: true,
      value: !!isMaster,
      writable: false
    });
  
    /**
     * A read-only property tells whether this rfr instance is a global master.
     * Call `require('rfr')` on a global rfr module to get a master rfr instance.
     * Using the global master is error-prone.
     * @type {Boolean}
     */
    Object.defineProperty(rfr, 'isGlobalMaster', {
      configurable: false,
      enumerable: true,
      value: constants.isGlobal && !!isMaster,
      writable: false
    });
  
    /**
     * The root of a rfr instance.
     * @type {String}
     */
    Object.defineProperty(rfr, 'root', {
      configurable: false,
      enumerable: true,
      get: function() {
        return callable.root;
      },
      set: function(root) {
        callable.root = coerceRoot(root);
      }
    });
  
    /**
     * Set the root.
     * @param {String} root The new root.
     */
    rfr.setRoot = function(root) {
      this.root = root;
    };
  
    /**
     * Get the filename that will be loaded when this rfr is called.
     * @returns {String} module filename.
     */
    rfr.resolve = function(idFromRoot) {
      return require.resolve(normalizeId(idFromRoot, this.root));
    };
  
    rfr.root = root;
    return rfr;
  }
docs: Create a new version of rfr with a function.
  @param {Function} callable The rfr require function.
  @param {String} root The root for rfr require.
  @param {Boolean} isMaster Whether this is a master rfr.
  @private