def boolean(string):
    """
    This returns True if the string is "True", False if "False",
    raises an Error otherwise. This is case insensitive.

    Args:
        string (str): A string specifying a boolean
    Returns:
        bool: Whether the string is True or False.
    """
    if string.lower() == 'true':
        return True
    elif string.lower() == 'false':
        return False
    else:
        raise ValueError("Expected string to be True or False")