Is `add!!(::Diagonal, ::Array)` correct?

[oxinabox]

Right now we claim that Diagonal is a inplacable_destination if it is wrapping around a vector that is itself an inplacable_destination.
However, Diagonal is only inplacable if the other side is itself diagonal.
Which one might in general assume to be the case since differential types should in general have same structure as primal types.
but IIRC there are some operations that have dense primal, but diagonal differentials.
So if you have that operation and also another function on the same data with a dense differntial then this could happen