Yes, this is because the contract enforces parametericity, which
doesn't allow `index-of` to work the way you'd want.
Instantiating things doesn't help because the contract is based on the
`require/typed`, not on the use site.
Here's a few suggestions:
1. Submit a pull-request to TR to add `index-of`
2. Import `index-of` at a concrete type
3. Use `unsafe-require/typed`
4. Use the third argument to `index-of?`
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