diff --git a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/ImmutableCollection.java b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/ImmutableCollection.java
index 2b5fc23b..8729323b 100644
--- a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/ImmutableCollection.java
+++ b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/ImmutableCollection.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 The Bank of New York Mellon.
+ * Copyright 2023 The Bank of New York Mellon.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -44,7 +44,11 @@ default ImmutableCollection<T> peek(Consumer<? super T> consumer)
     default <K> ImmutableBag<K> countBy(Function<? super T, ? extends K> function)
     {
         var counts = MutableBag.<K>empty();
-        this.forEach(each -> counts.add(function.apply(each)));
-        return counts.toImmutable();
+        return this.map(function, counts).toImmutable();
+    }
+
+    default <V> ImmutableBag<V> countByEach(Function<? super T, ? extends Iterable<V>> function)
+    {
+        return this.flatMap(function, MutableBag.empty()).toImmutable();
     }
 }
diff --git a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableBag.java b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableBag.java
index b55ec988..cd94b694 100644
--- a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableBag.java
+++ b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableBag.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 The Bank of New York Mellon.
+ * Copyright 2023 The Bank of New York Mellon.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -79,44 +79,27 @@ default MutableBag<T> peek(Consumer<? super T> consumer)
     default MutableBag<T> filter(Predicate<? super T> predicate)
     {
         var mutableBag = MutableBag.<T>empty();
-        for (T each : this)
-        {
-            if (predicate.test(each))
-            {
-                mutableBag.add(each);
-            }
-        }
-        return mutableBag;
+        return this.filter(predicate, mutableBag);
     }
 
     @Override
     default MutableBag<T> filterNot(Predicate<? super T> predicate)
     {
-        var mutableBag = MutableBag.<T>empty();
-        for (T each : this)
-        {
-            if (!predicate.test(each))
-            {
-                mutableBag.add(each);
-            }
-        }
-        return mutableBag;
+        return this.filter(predicate.negate());
     }
 
     @Override
     default <V> MutableBag<V> map(Function<? super T, ? extends V> function)
     {
         var mutableBag = MutableBag.<V>empty();
-        this.forEach(each -> mutableBag.add(function.apply(each)));
-        return mutableBag;
+        return this.map(function, mutableBag);
     }
 
     @Override
     default <V> MutableBag<V> flatMap(Function<? super T, ? extends Iterable<V>> function)
     {
         var mutableBag = MutableBag.<V>empty();
-        this.forEach(each -> mutableBag.addAllIterable(function.apply(each)));
-        return mutableBag;
+        return this.flatMap(function, mutableBag);
     }
 
     default <K> ArrayListMultimap<K, T> groupBy(Function<? super T, ? extends K> function)
diff --git a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableCollection.java b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableCollection.java
index d0f1c892..46b07977 100644
--- a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableCollection.java
+++ b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableCollection.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 The Bank of New York Mellon.
+ * Copyright 2023 The Bank of New York Mellon.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,15 +29,12 @@ default boolean addAllIterable(Iterable<T> iterable)
         {
             return this.addAll(collection);
         }
-        else
+        boolean result = false;
+        for (T each : iterable)
         {
-            boolean result = false;
-            for (T each : iterable)
-            {
-                result |= this.add(each);
-            }
-            return result;
+            result |= this.add(each);
         }
+        return result;
     }
 
     @Override
@@ -71,4 +68,9 @@ default <K> MutableBag<K> countBy(Function<? super T, ? extends K> function)
         this.forEach(each -> counts.add(function.apply(each)));
         return counts;
     }
+
+    default <V> MutableBag<V> countByEach(Function<? super T, ? extends Iterable<V>> function)
+    {
+        return this.flatMap(function, MutableBag.empty());
+    }
 }
diff --git a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableList.java b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableList.java
index 09a5fb33..6c2d02ba 100644
--- a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableList.java
+++ b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableList.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 The Bank of New York Mellon.
+ * Copyright 2023 The Bank of New York Mellon.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -81,44 +81,27 @@ default MutableList<T> asUnmodifiable()
     default MutableList<T> filter(Predicate<? super T> predicate)
     {
         var mutableList = MutableList.<T>empty();
-        for (T each : this)
-        {
-            if (predicate.test(each))
-            {
-                mutableList.add(each);
-            }
-        }
-        return mutableList;
+        return this.filter(predicate, mutableList);
     }
 
     @Override
     default MutableList<T> filterNot(Predicate<? super T> predicate)
     {
-        var mutableList = MutableList.<T>empty();
-        for (T each : this)
-        {
-            if (!predicate.test(each))
-            {
-                mutableList.add(each);
-            }
-        }
-        return mutableList;
+        return this.filter(predicate.negate());
     }
 
     @Override
     default <V> MutableList<V> map(Function<? super T, ? extends V> function)
     {
         var mutableList = MutableList.<V>empty();
-        this.forEach(each -> mutableList.add(function.apply(each)));
-        return mutableList;
+        return this.map(function, mutableList);
     }
 
     @Override
     default <V> MutableList<V> flatMap(Function<? super T, ? extends Iterable<V>> function)
     {
         var mutableList = MutableList.<V>empty();
-        this.forEach(each -> mutableList.addAllIterable(function.apply(each)));
-        return mutableList;
+        return this.flatMap(function, mutableList);
     }
 
     @Override
@@ -135,6 +118,13 @@ default <K> MutableListMultimap<K, T> groupBy(Function<? super T, ? extends K> f
         return multimap;
     }
 
+    default <K> MutableListMultimap<K, T> groupByEach(Function<? super T, ? extends Iterable<K>> function)
+    {
+        var multimap = MutableListMultimap.<K, T>empty();
+        this.forEach(each -> function.apply(each).forEach(key -> multimap.put(key, each)));
+        return multimap;
+    }
+
     default <K> MutableListMultimap<K, T> groupByUnmodifiable(Function<? super T, ? extends K> function)
     {
         var multimap = this.<K>groupBy(function);
diff --git a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableSet.java b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableSet.java
index 6bacd4cb..60b55cde 100644
--- a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableSet.java
+++ b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/MutableSet.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 The Bank of New York Mellon.
+ * Copyright 2023 The Bank of New York Mellon.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -78,44 +78,27 @@ default java.util.Set<T> asUnmodifiable()
     default MutableSet<T> filter(Predicate<? super T> predicate)
     {
         var mutableSet = MutableSet.<T>empty();
-        for (T each : this)
-        {
-            if (predicate.test(each))
-            {
-                mutableSet.add(each);
-            }
-        }
-        return mutableSet;
+        return this.filter(predicate, mutableSet);
     }
 
     @Override
     default MutableSet<T> filterNot(Predicate<? super T> predicate)
     {
-        var mutableSet = MutableSet.<T>empty();
-        for (T each : this)
-        {
-            if (!predicate.test(each))
-            {
-                mutableSet.add(each);
-            }
-        }
-        return mutableSet;
+        return this.filter(predicate.negate());
     }
 
     @Override
     default <V> MutableSet<V> map(Function<? super T, ? extends V> function)
     {
         var mutableSet = MutableSet.<V>empty();
-        this.forEach(each -> mutableSet.add(function.apply(each)));
-        return mutableSet;
+        return this.map(function, mutableSet);
     }
 
     @Override
     default <V> MutableSet<V> flatMap(Function<? super T, ? extends Iterable<V>> function)
     {
         var mutableSet = MutableSet.<V>empty();
-        this.forEach(each -> mutableSet.addAllIterable(function.apply(each)));
-        return mutableSet;
+        return this.flatMap(function, mutableSet);
     }
 
     default <K, V> MutableSetMultimap<K, T> groupBy(Function<? super T, ? extends K> function)
diff --git a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/RichIterable.java b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/RichIterable.java
index 5c617e9d..09758ce3 100644
--- a/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/RichIterable.java
+++ b/deck-of-cards-kata/src/main/java/bnymellon/codekatas/deckofcards/custom/collections/RichIterable.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 The Bank of New York Mellon.
+ * Copyright 2023 The Bank of New York Mellon.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,11 @@ public interface RichIterable<T> extends Iterable<T>
 
     boolean isEmpty();
 
+    default boolean notEmpty()
+    {
+        return !this.isEmpty();
+    }
+
     default Stream<T> stream()
     {
         throw new UnsupportedOperationException();
@@ -39,12 +44,41 @@ default Stream<T> stream()
 
     RichIterable<T> filter(Predicate<? super T> predicate);
 
+    default <R extends Collection<T>> R filter(Predicate<? super T> predicate, R target)
+    {
+        this.forEach(each ->
+        {
+            if (predicate.test(each))
+            {
+                target.add(each);
+            }
+        });
+        return target;
+    }
+
     RichIterable<T> filterNot(Predicate<? super T> predicate);
 
+    default <R extends Collection<T>> R filterNot(Predicate<? super T> predicate, R target)
+    {
+        return this.filter(predicate.negate(), target);
+    }
+
     <V> RichIterable<V> map(Function<? super T, ? extends V> function);
 
+    default <V, R extends Collection<V>> R map(Function<? super T, ? extends V> function, R target)
+    {
+        this.forEach(each -> target.add(function.apply(each)));
+        return target;
+    }
+
     <V> RichIterable<V> flatMap(Function<? super T, ? extends Iterable<V>> function);
 
+    default <V, R extends MutableCollection<V>> R flatMap(Function<? super T, ? extends Iterable<V>> function, R target)
+    {
+        this.forEach(each -> target.addAllIterable(function.apply(each)));
+        return target;
+    }
+
     default RichIterable<T> peek(Consumer<? super T> consumer)
     {
         this.forEach(consumer);
@@ -80,6 +114,11 @@ default boolean anyMatch(Predicate<? super T> predicate)
         return false;
     }
 
+    default <V> boolean containsBy(Function<? super T, ? extends V> function, V value)
+    {
+        return this.anyMatch(each -> value.equals(function.apply(each)));
+    }
+
     default boolean allMatch(Predicate<? super T> predicate)
     {
         for (T each : this)
@@ -147,10 +186,19 @@ default MutableSet<T> toSet()
         return this.toCollection(MutableSet::empty, Collection::add);
     }
 
+    default MutableBag<T> toBag()
+    {
+        return this.toCollection(MutableBag::empty, Collection::add);
+    }
+
     default <R extends Collection<T>> R toCollection(Supplier<R> supplier, BiConsumer<R, T> addMethod)
     {
         R collection = supplier.get();
         this.forEach(each -> addMethod.accept(collection, each));
         return collection;
     }
+
+    <K> Bag<K> countBy(Function<? super T, ? extends K> function);
+
+    <V> Bag<V> countByEach(Function<? super T, ? extends Iterable<V>> function);
 }
diff --git a/deck-of-cards-kata/src/test/java/bnymellon/codekatas/deckofcards/custom/collections/MutableListTest.java b/deck-of-cards-kata/src/test/java/bnymellon/codekatas/deckofcards/custom/collections/MutableListTest.java
index b01845ee..bee34215 100644
--- a/deck-of-cards-kata/src/test/java/bnymellon/codekatas/deckofcards/custom/collections/MutableListTest.java
+++ b/deck-of-cards-kata/src/test/java/bnymellon/codekatas/deckofcards/custom/collections/MutableListTest.java
@@ -22,6 +22,7 @@
 import java.util.Set;
 import java.util.stream.Collector;
 import java.util.stream.Collectors;
+import java.util.stream.IntStream;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -412,4 +413,43 @@ public void addAllIterable()
         list2.addAllIterable(expected::iterator);
         Assertions.assertEquals(expected, list2);
     }
+
+    @Test
+    public void countByEach()
+    {
+        MutableList<MutableList<Integer>> list = MutableList.of(MutableList.of(1, 2), MutableList.of(1, 2, 3));
+        MutableBag<Integer> counts = list.countByEach(each -> each);
+        Assertions.assertEquals(2, counts.getOccurrences(1));
+        Assertions.assertEquals(2, counts.getOccurrences(2));
+        Assertions.assertEquals(1, counts.getOccurrences(3));
+    }
+
+    @Test
+    public void groupByEach()
+    {
+        MutableList<Integer> list = MutableList.of(1, 2, 3);
+        MutableListMultimap<Integer, Integer> multimap = list.groupByEach(each -> IntStream.range(0, each).boxed().toList());
+        Assertions.assertEquals(List.of(1, 2, 3), multimap.get(0));
+        Assertions.assertEquals(List.of(2, 3), multimap.get(1));
+        Assertions.assertEquals(List.of(3), multimap.get(2));
+    }
+
+    @Test
+    public void toBag()
+    {
+        MutableList<Integer> list = MutableList.of(1, 2, 2, 3, 3, 3);
+        MutableBag<Integer> bag = list.toBag();
+
+        Assertions.assertEquals(MutableBag.of(1, 2, 2, 3, 3, 3), bag);
+    }
+
+    @Test
+    public void containsBy()
+    {
+        MutableList<Integer> hasEvens = MutableList.of(1, 2, 3);
+        MutableList<Integer> onlyOdds = MutableList.of(1, 3);
+
+        Assertions.assertTrue(hasEvens.containsBy(each -> each % 2, 0));
+        Assertions.assertFalse(onlyOdds.containsBy(each -> each % 2, 0));
+    }
 }