public function mapWithKey( (function(Tk,Tv):Tu) $fn, ): KeyedIterable<Tk,Tu>
Every key and value in the current
KeyedIterable is affected by a call to
filterWithKey() where only values that meet a
certain criteria are affected.
(function(Tk,Tv):Tu) $fn- The callback containing the operation to apply to the
KeyedIterablekeys and values.