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building/tooling/test-runners/libraries.md

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+# Libraries
+
+Exercism aims to teach [fluency](/docs/using/product/fluency) in the syntax, idioms, and the standard library of the language.
+This explicitly does _not_ mention external (non built-in) libraries, as that falls under _proficiency_.
+Exercises must thus, with few exceptions, be solvable without using libraries.
+
+## Reasons to support libraries
+
+There are couple of reasons why a track might still want to support using libraries:
+
+1. The language has a minimal standard library and it is idiomatic to use libraries to add functionality.
+   An example of such a language is Rust.
+2. The exercise is only really solvable using libraries.
+   An example of such an exercise is the [lens-person exercise](https://exercism.org/exercises/lens-person), which in many languages can only be solved using a library.
+3. The library adds testing functionality.
+   An example is a library that adds support for property-based testing.
+
+## Supporting libraries in the test runner
+
+As the test runner does not have access to the internet whilst running, it is not possible to download libraries at run time.
+The **only** solution to this problem is to install/download libraries at build time, where you _do_ have access to the internet.
+In practice, this means you'll need to install/download libraries within the Dockerfile.
+
+https://github.com/exercism/prolog-test-runner/blob/ed7447a7518ede6ee3405e649f50aaec828e318b/Dockerfile
+
+## Documentation
+
+If your track supports additional
+(/docs/building/tracks/docs)
+We recommend creating a document in the track repo's `docs` directory that lists the libraries that people can use, po