function clbk( error ) {
		if ( error ) {
			debug( 'Encountered an error: %s', error.message );
			return done( error );
		}
		count += 1;
		debug( 'Completed invocation %d of %d.', count, n );
		if ( idx < m ) {
			return next();
		}
		if ( count === n ) {
			debug( 'Finished invoking a function.' );
			return done( null, out );
		}
	}
Callback invoked once ready to process the next invocation.

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