code: function createToolingApi( listeners ) {
     assert( listeners ).hasType( Array );
  
     return {
        /**
         * Register a function to be called when the composition of active observed items changes.
         *
         * @param {Function} callback
         *    a function to call with updated debug information
         * @return {AxTooling}
         *    the tooling instance
         * @memberof AxTooling
         */
        onChange( callback ) {
           listeners.push( callback );
           return this;
        },
  
        /**
         * Unsubscribe a registered {@link #AxTooling.onChange} callback
         *
         * @param {Function} callback
         *    a function that was previously passed to {@link #AxTooling.onChange}
         * @return {AxTooling}
         *    the tooling instance
         * @memberof AxTooling
         */
        unsubscribe( callback ) {
           remove( listeners, callback );
           return this;
        },
  
        /**
         * A {@link PagesTooling} interface to the {@link AxTooling} instance.
         *
         * @type {PagesTooling}
         * @memberof AxTooling
         * @deprecated
         */
        pages: null,
  
        release: () => {}
     };
  }
docs: ////////////////////////////////////////////////////////////////////////////////////////////////////////////