protected function fetch()
    {
        return Collection::make(file($this->source, FILE_SKIP_EMPTY_LINES | FILE_IGNORE_NEW_LINES))
            ->map(function ($line) {
                // capture everything between single quotes
                preg_match_all(self::PATTERN_CAPTURE, $line, $matches);
                // matches[x]
                //   0 - complete match
                //   1 - captures
                return $matches[ 1 ];
            })->filter();
    }