def _B36Check(b36val):
    """
    Verify that a string is a valid path base-36 integer.

    >>> _B36Check('Aa')
    'aa'

    >>> _B36Check('.')
    Traceback (most recent call last):
        ...
    ValueError: invalid ...
    """
    int(b36val, 36)
    return str(b36val).lower()