def letters_only(word:str, lower_case:bool = True):
    """
    Return only letters of string input parameter

    Parameters
    ----------
    word : str
        String from which letters are being returned
    lower_case : bool
        If set to True (default) return lower-case letters
    
    Returns
    -------
    letters : str
        (Lower-case) letter elements of input string
    """
    lo = ''.join([l for l in word if l.lower() in 'abcdefghijklmnopqrstuvwxyz'])
    if lower_case:
        lo = lo.lower()
    return lo