def groupletters(x, _low=lower_function):
    """
    Double all characters, making doubled letters lowercase.

    Parameters
    ----------
    x : str

    Returns
    -------
    str

    Examples
    --------

        >>> groupletters("Apple")
        {u}'aAppppllee'

    """
    return "".join(ichain.from_iterable((_low(y), y) for y in x))
Double all characters, making doubled letters lowercase.

    Parameters
    ----------
    x : str

    Returns
    -------
    str

    Examples
    --------

        >>> groupletters("Apple")
        {u}'aAppppllee'