Following the sequence of code in the book, when I enter:
val immu = Map.empty ++ muta

the response is:
immu: scala.collection.mutable.Map[String,Int] = Map(ii -> 2, i -> 1)

In order to get an immutable Map, I need to make the following changes:
import scala.collection.immutable
val immu = immutable.Map.empty ++ muta
---
Good point. Will add something to clarify need to restart the REPL after
that import of scala.collection.mutable.Map. Tnx.