Implementing CustomScan over an index

I've built a custom type of index. I implemented an index access method, but have run into roadblocks. I need to:

1. See the other quals in the where clause.
2. Extract some things from the projection.
3. Insert some things in the projection.
4. Push some aggregations down into the index.

So I started implementing a CustomScan. It's not trivial.

I've learned that the system executes ExecInitCustomScan automatically, but I probably need it to do most of the stuff in ExecInitIndexScan, and then execute the scan mostly the way it's done in IndexNext.

Basically, I want just a normal index scan, but with the ability to do custom things with the quals and the projection.

So... what's the best approach?

Is there any sample code that does this? A search of github doesn't turn up much.

Is there any way to do this without duplicating everything in nodeIndexscan.c myself?

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