This will save.. five comparisons.. occasionally? But it's a more
transparent design, and it seems more charitable to group interface
changes together.
Don't redundantly readIO/parse the rulesets; do this once, lazily
carrying out the involved operations. Rulesets are invariant over
executions.
This improves performance by a few orders of magnitude. Though at some
point we should substitute linear search for lookup on a generalised
suffix tree of rooted domains.
Breaking interface change, though I'll likely restore the old form
soon with IO TH.
vi