In reply to @enthus1ast. "Conceptually I get them": I mean I the `NifCursor`
and `NifBuilder` or `Replacer` works
Since they are essentially token streams, the cursor tells you where you are in
the current tree.
Each element is either a `TagLit` (able to contains node) or a `Leaf` (Symbol,
DotToken)
Each tag encode how many children it has, allowing an AST like traversal on a
linear buffer

Then the `NifBuilder` is just about building a new tree from the old one