Correct `Params` instances for `lookup` and `singletonM`.
We always use Leibniz keys (nat for list, Countable for maps), so the Params should be one higher.
In fact, for singletonM and insert we already use the correct number for Params.
We always use Leibniz keys (nat for list, Countable for maps), so the Params should be one higher.
In fact, for singletonM and insert we already use the correct number for Params.