code: function( rootId ) { //jshint ignore:line
          var lib = {};
          for ( var libname in libraryFactories ) {
              if ( libraryFactories.hasOwnProperty( libname ) ) {
                  if ( lib[ libname ] !== undefined ) {
                      throw "impress.js ERROR: Two libraries both tried to use libname: " +  libname;
                  }
                  lib[ libname ] = libraryFactories[ libname ]( rootId );
              }
          }
          return lib;
      }
docs: Call each library factory, and return the lib object that is added to the api.