function callbackFactory( username, idx ) {
		return onResponse;
		/**
		* Callback invoked upon receiving a request response.
		*
		* @private
		* @param {(Error|null)} error - error object
		* @param {Object} details - response data
		* @param {Object} info - rate limit info
		* @returns {void}
		*/
		function onResponse( error, details, info ) {
			if ( eFLG ) {
				return;
			}
			debug( 'Response received for username: `%s` (%d).', username, idx );
			if ( arguments.length === 1 ) {
				eFLG = true;
				debug( 'Encountered an application-level error for username `%s` (%d): %s', username, idx, error.message );
				return clbk( error );
			}
			ratelimit = updateRateLimit( ratelimit, info );
			if ( error ) {
				debug( 'Failed to resolve username `%s` (%d): %s', username, idx, error.message );
				out.failures[ username ] = error.message;
				fcount += 1;
			} else {
				debug( 'Successfully resolved username `%s` (%d).', username, idx );
				out.data[ username ] = details;
				scount += 1;
			}
			count += 1;
			debug( 'Request %d of %d complete.', count, len );
			next();
		}
	}
Returns a response callback.

@private
@param {string} username - username
@param {number} idx - request index
@returns {Function} response callback