function next() {
		var j;
		idx += 1;
		j = idx;
		debug( 'Invocation number: %d', j );
		fcn.call( opts.thisArg, j, cb );

		/**
		* Callback invoked once a provided function completes.
		*
		* @private
		* @param {*} [error] - error
		* @param {*} [result] - result
		* @returns {void}
		*/
		function cb( error, result ) {
			if ( flg ) {
				// Prevent further invocations:
				return;
			}
			if ( error ) {
				flg = true;
				return clbk( error );
			}
			out[ j ] = result;
			clbk();
		}
	}
Callback to process the next function invocation.

@private