> And Julia provides no way of specifying the behaviour of abstract types. I'm not sure if "no way" is accurate. There are interfaces and one could use traits.

> And Julia provides no way of specifying the behaviour of abstract types. I'm not sure if "no way" is accurate. There are interfaces and one could use traits. As cited above, we could use `eachindex` or `CartesianIndices` to get a list of the valid indices. The problem is enforcing and testing these interfaces.