use GFM fenced code-blocks for code
Don't refer to classes without context until after they are introduced.

At the point in text where this occurs, we are attempting to explain the
most basic building block in terms of more-complex, yet-to-be-introduced ones.
I think a more-simplistic view is helpful for beginners here, and that we
can flesh out the details as we get to them.
Avoid overloading the term "classes"
Make conversation between "Us" & "Compi" read like a script; provide checkpoints.

Provide line-by-line checkpoints so the sudent doesn't get lost in the banter,
and provide the structure of a screenplay script to help the student follow.
Remove reference to example removed in c4347b91d9
minor clarifications