I don't know what I was originally thinking, here - there's no way it
could ever have been correct to insert the original path into the trie.
The additional subpath caused only cached results to be incorrect, as
subsequent calls would find the longer/subpath in the trie, and return
that out erroneously as a root.

This also changes all deduction tests to run twice in order to ensure
we're testing both the cached and uncached paths.

Fixes #217.