> {-# LANGUAGE NoMonomorphismRestriction #-}
> 
> import Diagrams.Prelude

We make use of a tree layout module from the `diagrams-contrib` package:

> import Diagrams.TwoD.Layout.Tree

Generate the tree of naive Fibonacci calls.

> fibCalls :: Integer -> BTree Integer
> fibCalls 0 = leaf 0
> fibCalls 1 = leaf 1
> fibCalls n = BNode n (fibCalls (n-1)) (fibCalls (n-2))

Lay out the tree and render it by providing a function to render nodes
and a function to render edges.

> Just t = uniqueXLayout 2 2 (fibCalls 5)
>       
> example = pad 1.1 . lw 0.03 . centerXY 
>         $ renderTree 
>             (\n -> (text ("fib " ++ show n)
>                     <> roundedRect 3 1.3 0.3 # fc white)
>             )
>             (~~) t