Add mutable.SeqMap plus make LinkedHashMap extend it

Author: mdedetrich

library/src/scala/collection/immutable/SeqMap.scala

@@ -19,7 +19,7 @@ import scala.collection.mutable.{Builder, ImmutableBuilder}
   * @author Matthew de Detrich
   * @version 2.13
   * @since 2.13
-  * @define coll immutable linked map
+  * @define coll immutable seq map
   * @define Coll `immutable.SeqMap`
   */
 

library/src/scala/collection/mutable/LinkedHashMap.scala

@@ -46,6 +46,7 @@ object LinkedHashMap extends MapFactory[LinkedHashMap] {
  */
 class LinkedHashMap[K, V]
   extends AbstractMap[K, V]
+    with SeqMap[K, V]
     with MapOps[K, V, LinkedHashMap, LinkedHashMap[K, V]]
     with StrictOptimizedIterableOps[(K, V), Iterable, LinkedHashMap[K, V]] {
 

library/src/scala/collection/mutable/SeqMap.scala

@@ -0,0 +1,27 @@
+package scala
+package collection
+package mutable
+
+import java.io.{ObjectInputStream, ObjectOutputStream}
+
+import scala.collection.mutable.{Builder, ImmutableBuilder}
+
+/**
+  * A generic trait for ordered mutable maps. Concrete classes have to provide
+  * functionality for the abstract methods in `SeqMap`.
+  *
+  * Note that when checking for equality [[SeqMap]] does not take into account
+  * ordering.
+  *
+  * @tparam K      the type of the keys contained in this linked map.
+  * @tparam V      the type of the values associated with the keys in this linked map.
+  *
+  * @author Matthew de Detrich
+  * @version 2.13
+  * @since 2.13
+  * @define coll mutable Seq map
+  * @define Coll `mutable.SeqMap`
+  */
+
+trait SeqMap[K, V] extends AbstractMap[K, V]
+  with MapOps[K, V, SeqMap, SeqMap[K, V]]