Read this post from the bottom up; the editor is placing new-lines in the wrong place...
The “Unresolved reference: MutableSet” type errors don’t help any in working this out (saying “MutableSet is a trait; objects can only be created from non-abstract classes” would be so much clearer).
It seems that jet.MutableSet is a trait which is satisfied by java.util.TreeSet, etc. Is the only point to expose a Kotlin-ised API and make usage of immutable containers (or at least mutable containers hidden behind immutable interfaces) easier?
Where is the documentation for Kotlin’s collection classes? They’re not mentioned in the API documentation nor much in any other documentation that I can see.