def one(item):
    """
    Return the first element from the iterable, but raise an exception
    if elements remain in the iterable after the first.

    >>> one(['val'])
    'val'

    >>> one(['val', 'other'])
    Traceback (most recent call last):
    ...
    ValueError: ...values to unpack...

    >>> one([])
    Traceback (most recent call last):
    ...
    ValueError: ...values to unpack...

    >>> numbers = itertools.count()
    >>> one(numbers)
    Traceback (most recent call last):
    ...
    ValueError: ...values to unpack...
    >>> next(numbers)
    2
    """
    (result,) = item
    return result