I meant something like this:
```scala
/* We assume we own this mutable map, which is to say, mut is the only reference to it */
override def  fromMutable(mut: MMap[K, V]) = new collection.immutable.Map[K, V] {
  /* only copy on write*/
  def +[V1>:V](kv: (K,V1)) = mut.toMap + kv
  def -(k: K) = mut.toMap - k
  /* otherwise wrap */
  def get(k: K) = mut.get(k)
  def iterator = mut.iterator
}
```
similarly for scala.collection.Map