experimental: try using doctest

I'm finding that many examples should be tests and many tests should be examples. The [doctest](https://github.com/hughjonesd/doctest/) idea is common in many ecosystems for this reason and that package enables that for R. Would be interesting to give it a shot and see if it helps.

I'm not convinced that this will be feasible - sliding operations are necessarily quite delicate and particular - but maybe it will be a handy forcing function.

Start by reviewing the repository's existing examples and tests, then read the doctest package documentation to assess how it could be used with R. Check whether the sliding operations can be represented reliably as doctests. Done means determining whether the experiment is feasible and documenting or implementing a workable approach.

Written by the indexing model from the issue text.