src/Russian/RussianLanguage.php
public static function in($word)
    {
        $normalized = trim(S::lower($word));
        if (in_array(S::slice($normalized, 0, 1), ['в', 'ф'], true))
            return 'во '.$word;
        return 'в '.$word;
    }
public static function in($word)
    {
        $normalized = trim(S::lower($word));
        if (in_array(S::slice($normalized, 0, 1), ['в', 'ф'], true))
            return 'во '.$word;
        return 'в '.$word;
    }
Add 'в' or 'во' prepositional before the word
@param string $word
@return string