src/Russian/RussianLanguage.php
public static function with($word)
    {
        $normalized = trim(S::lower($word));
        if (in_array(S::slice($normalized, 0, 1), ['c', 'з', 'ш', 'ж'], true) && static::isConsonant(S::slice($normalized, 1, 2)) || S::slice($normalized, 0, 1) == 'щ')
            return 'со '.$word;
        return 'с '.$word;
    }
public static function with($word)
    {
        $normalized = trim(S::lower($word));
        if (in_array(S::slice($normalized, 0, 1), ['c', 'з', 'ш', 'ж'], true) && static::isConsonant(S::slice($normalized, 1, 2)) || S::slice($normalized, 0, 1) == 'щ')
            return 'со '.$word;
        return 'с '.$word;
    }
Add 'с' or 'со' prepositional before the word
@param string $word
@return string