from typing import Optional


def boolean_type(value: str) -> Optional[bool]:
    """
    Converts a string value into a boolean value, if possible.

    The string is converted into True, if its lowercase value equals '1', 'true' or 't'.
    It is converted into False, if its lowercase value equals '0', 'false' or 'f'.
    If value equals non of these values, None is returned.

    Parameters
    ----------
    value : str
        The string value to be casted into a boolean value

    Returns
    -------
    Optional[bool]
        True or False regarding the rules mentioned above, None otherwise
    """

    if value.lower() in ('1', 'true', 't'):
        return True
    if value.lower() in ('0', 'false', 'f'):
        return False