def parse_multi_keys(item: str, sep: None | str = None) -> str:
    """Parse keys that contain one or more spaces (see **sep**).

    The first word in the to-be returned key is decapitalized.

    Examples
    --------
    .. code:: python

        >>> out = parse_multi_keys('A B')
        >>> print(repr(out))
        'a B'

    Parameters
    ----------
    item : :class:`str`
        A string containing at least a single space.
    sep : :class:`str`, optional
        The delimiter according which to split **item**.
        ``None`` (the default value) means split according to any whitespace,
        and discard empty strings from the result.

    Returns
    -------
    :class:`str`
        **item** with its first word decapitalized.

    Raises
    ------
    ValueError
        Raised if **item** does not contain any whitespaces.

    """
    sep = sep if sep is not None else ' '
    i1, i2 = item.split(sep, maxsplit=1)
    i1 = i1.lstrip('&').lower()
    return sep.join((i1, i2))