Multihot cotangents WIP (doesn't work)multihot-cotangents

The idea is sound but for a smaller source language. Notes also in
Obsidian, but the theory so far is that dropping support for nested
arrays makes this possible, although making the result type-safe (i.e.
not have partial functions in a bunch of places) would require making
the lack of nested array support explicit in the embedded type system,
i.e. have Accelerate-like stratification.

The point is that multihots can be added heterogeneously using
plusSparseS but not homogeneously with EPlus or plusSparse, because the
indices might differ between the summands. Thus as long as we never need
to homogeneously sum multihot cotangents, we're golden.

Now the crucial observation is that we only need plus to be homogeneous
on array elements. So if array elements cannot themselves be arrays,
i.e. we drop support for nested arrays, no homogeneous plus of multihot
array cotangents is needed, and we can have static multihots.

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