public function add($prefix, $dir, $prepend = false)
    {
        $prefix = trim($prefix, '\\') . '\\';
        $dir = rtrim($dir, DIRECTORY_SEPARATOR) . '/';

        if (isset($this->prefixes[$prefix]) === false) {
            $this->prefixes[$prefix] = [];
        }

        if ($prepend) {
            array_unshift($this->prefixes[$prefix], $dir);
        } else {
            array_push($this->prefixes[$prefix], $dir);
        }

        return $this;
    }