def icontains(header_value: str, rule_value: str) -> bool:
    """
    Case insensitive implementation of str-in-str lookup.

    Parameters
    ----------
    header_value : str
        String to look within.
    rule_value : str
        String to look for.

    Returns
    -------
    bool
        Whether *rule* exists in *value*
    """
    return rule_value.lower() in header_value.lower()