function parallel() {
	var options;
	var files;
	var opts;
	var clbk;
	var err;
	var dir;
	var i;

	files = arguments[ 0 ];
	if ( !isStringArray( files ) ) {
		throw new TypeError( format( 'invalid argument. First argument must be an array of strings. Value: `%s`.', files ) );
	}
	files = files.slice();
	opts = defaults();
	if ( arguments.length > 2 ) {
		options = arguments[ 1 ];
		clbk = arguments[ 2 ];
		err = validate( opts, options );
		if ( err ) {
			throw err;
		}
	} else {
		clbk = arguments[ 1 ];
	}
	if ( !isFunction( clbk ) ) {
		throw new TypeError( format( 'invalid argument. Callback argument must be a function. Value: `%s`.', clbk ) );
	}
	// Prevent the number of concurrent scripts exceeding the number of actual scripts to run.
	if ( opts.concurrency > files.length ) {
		opts.concurrency = files.length;
	}
	// Prevent the number of workers exceeding the number of concurrent scripts (excess capacity), as some workers would never be allocated scripts to run and always be idle.
	if ( opts.workers > opts.concurrency ) {
		opts.workers = opts.concurrency;
	}
	// Resolve any relative paths to absolute paths...
	dir = cwd();
	for ( i = 0; i < files.length; i++ ) {
		files[ i ] = path.resolve( dir, files[ i ] );
	}
	exec( files, opts, done );

	/**
	* Callback invoked after executing all scripts.
	*
	* @private
	* @param {Error} error - error object
	* @returns {void}
	*/
	function done( error ) {
		if ( error ) {
			return clbk( error );
		}
		clbk();
	}
}
Executes scripts in parallel.

@param {StringArray} files - script file paths
@param {Options} [options] - function options
@param {string} [options.cmd='node'] - executable file/command
@param {PositiveInteger} [options.concurrency] - number of scripts to execute concurrently
@param {PositiveInteger} [options.workers] - number of workers
@param {boolean} [options.ordered=false] - boolean indicating whether to preserve the order of script output
@param {NonNegativeInteger} [options.uid] - process user identity
@param {NonNegativeInteger} [options.gid] - process group identity
@param {NonNegativeInteger} [options.maxBuffer=200*1024*1024] - max child process `stdio` buffer size
@param {Callback} clbk - callback to invoke after executing all scripts
@throws {TypeError} first argument must be an array of strings
@throws {TypeError} options argument must be an object
@throws {TypeError} must provide valid options
@throws {TypeError} callback argument must be a function

@example
var files = [ './a.js', './b.js ' ];

var opts = {
    'workers': 3,
    'concurrency': 3
};

function done( error ) {
    if ( error ) {
        throw error;
    }
}

parallel( files, opts, done );