def remove_numbers(doc: str) ->str:
    """Removes digit/number characters from a string.

    Parameters
    ----------
    doc: str
        A document given as a single string.

    Returns
    -------
    str
        The document as a single string with all number/digit characters
        removed.

    """
    removed = filter(lambda character: not character.isdigit(), str(doc))
    return ''.join(removed)
