function next() {
		var i;

		idx += 1;

		// Cache the current index value to allow storing results later:
		i = idx;

		fcns[ idx ].call( opts.thisArg, resolve );

		/**
		* Callback invoked once a provided function finishes.
		*
		* @private
		* @param {*} [error] - error
		* @param {*} [results] - results
		* @returns {void}
		*/
		function resolve( error, results ) {
			if ( flg ) {
				// Prevent further processing:
				return;
			}
			if ( error ) {
				flg = true;
				return clbk( error );
			}
			out[ i ] = results;
			clbk();
		}
	}
Callback to invoke the next function.

@private