Indeed, the parent already gives one: flip_case(flip_case("ff")) = "ff". (Since it's hard to tell with what I guess is default ligature formation, at least in my

Indeed, the parent already gives one: flip_case(flip_case("ff")) = "ff". (Since it's hard to tell with what I guess is default ligature formation, at least in my browser, the first is an 'ff' ligature and the second is two 'f's.)