function clbk( error ) {
		if ( error ) {
			debug( 'Encountered an error: %s', error.message );
			return done( error );
		}
		count += 1;
		debug( 'Processed %d of %d collection elements.', count, len );
		if ( idx < maxIndex ) {
			return next();
		}
		if ( count === len ) {
			debug( 'Finished processing a collection.' );
			return done( null, out );
		}
	}
Callback invoked once ready to process the next collection element.

@private
@param {*} [error] - error
@returns {void}