An assistant to understanding: MapAt or ReplaceAll a Button to prominently display what targets are hit by your operation.
In the last section, toward the end I suggested a
Hold-like wrapper that would display itself as a frame, which is less obtrusive visually than
Hold[...] wrapped around everything in the expression.
In that vein, here is a technique that sometimes helps in debugging. Use
ReplaceAll or other operations to wrap
Button, or some other prominently visible markup, around certain targets, especially if you're not sure what the targets will be.