KEEP #23: Update list of open questions, provide resolutions to some …

…of them.

proposals/stdlib/group-and-fold.md

@@ -189,14 +189,24 @@ The operation returns `Map<K, R>`, where `K` is the type of the key and `R` is t
 
     * `count`/`sumBy` can be misinterpreted as operations on the whole collection, rather on each group.
         * resolution: `countEach`, `sumEachBy`
+
+    * `Grouping` is often referenced in docs as "grouping source", so maybe we should name it `GroupingSource`?
 
 2. Which general forms of `fold`/`reduce`/`aggregate` should we provide?
     * Method count is increased
+        * resolution: since there's a single receiver for all operations, the impact on method count is not that big.
     * Having them as overloads hurts completion during gradual typing.
 3. Should we provide `To`-overloads (like `groupByTo`) with a mutable map as a target parameter?
+    * resolution: there are no easy alternatives to achieve the same results without these overloads,
+     so we should provide them.
+     
 4. Having primitive fold accumulators stored in a map introduces a lot of boxing.
-
-    * resolution: provide optimized implementations of `countEach` and `sumEachBy`.
+    * resolution: provide optimized implementations of `countEach` and `sumEachBy`. 
+    Note that, there's no such optimization for `countEachTo` and `sumEachByTo`.
+
+5. Should we provide `sumEachByLong` and `sumEachByDouble` operations, 
+or wait until [KT-11265](https://youtrack.jetbrains.com/issue/KT-11265) is resolved, 
+so we could have them as overloads of `sumEachBy`?
 
 ## Future advancements