Correct encoding of this existential type in Scala?

First off Step looks perfect to me. As for Stream I think you're on the right track with the abstract type. Here's what I came up with (including implementations of the remaining methods in section 2.1 of the Coutts paper).

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.