[foldr]

Ah, I was going on what the OP said ("ensuring that a given pin cannot be used in multiple places").

That seems sensible, but also not particularly valuable. A lot of the time it makes sense both to 'read' and 'write' from a pin (e.g. if it's open-drain with a pullup).

[lulf]

This was an inaccuracy on my part, sorry for that. It should probably have been "... used in multiple places _at the same time_".