Agreed that perhaps the notion of walls I used was too narrowly focused on code modularity, but I think we're pretty much saying the same thing about isolation

Agreed that perhaps the notion of walls I used was too narrowly focused on code modularity, but I think we're pretty much saying the same thing about isolation and independence. The ability to apply local reasoning is the important part - if 'wall' is taken to mean something like this then I think we are in agreement.