docs: function makeMBTAPI( config ) {
    if ( ! config.apiKey || typeof config.apiKey !== 'string' ) {
      throw new Error( 'An MBTA API key must be provided' );
    }
    config.apiRoot = config.apiRoot || 'http://realtime.mbta.com/developer/api/v2/';
  
    return _.mapValues( methods, function( args, key ) {
      // makeQueryHandler's third argument is a config object: add it when applying
      return makeQueryHandler.apply( null, args.concat( config ) );
    });
  }