I've written a couple of objects that model data structures that we use here. I was just explaining the inner workings of one of them to the woman who is taking over from me. I'd forgotten quite how baroque it was.
The object is a reference to a hash of hashes of hashes. In each of these bottom level hashes, one of the values is a reference to a hash of arrays.
I drew a picture, but I'm not sure to what extent it helped! It's the data structure from hell.
Of course, the object's methods hide you from all this complexity. They just work