Improve instance search in `equation`

Meta `equation` finds the first appropriate instance, but it might not be the best. For example, it could use an instance of the Ring class instead of CRing if the instance of the former comes first.

Start by locating the implementation of the `equation` meta and its instance-search entry point; the payload names no files or tests. Determine how candidate instances are currently ordered and define what makes one a better match, then verify that `equation` selects the preferred applicable instance rather than simply the first one.

Written by the indexing model from the issue text.