Gaurav Arya

Good guess! I ran a bisect and the bug is caused by https://github.com/JuliaLang/julia/pull/45222.

On the other hand, it's not a word :/ OK this is just bikeshedding for now

https://en.wikipedia.org/wiki/Concrete_category#Definition

I think we should wait until the WOperator PR, for `isconvertible` to get some mileage in OrdinaryDiffEq. As I think there's still some more thinking to do on the semantics,...

if we support sparse block patterns, then block diagonal becomes a special case

Can always do `const BlockDiagonal = BlockOperator{AA} where {AA

Also want to be mindful of code dup with kronecker products. $I \otimes A$ is a block diagonal matrix, and $A \otimes B = (A \otimes I)(I \otimes B)$, although...

Edit: `L.op` already gets updated since it is part of `getops(L)`, so only the unused param task is left -- updated issue title accordingly. Again, we're going to hold off...

I'm also facing the issue of slow indexing when editing certain files, e.g. in `OrdinaryDiffEq` I have >1 minute wait time or even extension crashes after making a small edit...

I don't have the time right now to revisit this PR, but if it looks good and would be helpful, please feel free to fix anything remaining and merge. Thanks!