I realized that the example for prewalk/2 is a bit confusing since some people might assume that 5 + 3 * 7 is transformed to 5 * 3 + 7 while in reality it's transformed into something like 5 * (3 + 7) because the tree shape is not changed.

I think by showing the previous and new AST in the example can be more helpful for the developer.

I also added an example for prewalk/3 and included some references to these functions in the postwalk/2 and postwalk/3