def numerus(count, wordSingular, wordPlural=None):
    """
    Return the singular `wordSingular` or the plural form `wordPlural` of a
    noun, depending of the value of `count`.

    If `wordPlural` is `None`, it equals to `wordSingular` with an appended
    's'.

    >>> numerus(1, 'car')
    'car'

    >>> numerus(2, 'car')
    'cars'
    """

    if count == 1:
        return wordSingular
    else:
        if wordPlural is None:
            wordPlural = wordSingular + "s"
        return wordPlural