def validate_one(word, desc, partial=False):
    """
    validate_one(word, desc, partial=False)

    validate word against the constructed instance of the type
    in desc.  May raise exception.  If it returns false (and doesn't
    raise an exception), desc.instance.val will
    contain the validated value (in the appropriate type).
    """
    desc.instance.valid(word, partial)
    desc.numseen += 1
    if desc.N:
        desc.n = desc.numseen + 1
