`Range` polyfill conflicts with other polyfills

Using `IX.Observable` pulls in `IX.Library` as a dependency, and `Range` is `public` there.
It looks like you've tried to make it conditional on the TFM? Unfortunately that's not sufficient. If you're not using `Range` in the public API surface, consider adopting https://github.com/Sergio0694/PolySharp or https://github.com/meziantou/Meziantou.Polyfill for generated, `internal` polyfills.