    "A function that returns the result of applying the given
     [[function|resulting]] to the [[key|Entry.key]] of
     a given [[Entry]], discarding its `item`.

         Map<String,List<Item>> map = ... ;
         {String*} uppercaseKeys = map.map(forKey(String.uppercased));"
    see (`function forItem`)
    tagged("Functions")
    shared Result forKey<Key,Result>(Result resulting(Key key))
                (Key->Anything entry)
            given Key satisfies Object
            => resulting(entry.key);