The negation here is confusing. The intent of the code is "if `s1` is preferred
over `s2` as the canonical `SigBit` for this signal, make `s1` the canonical `SigBit`
in `assign_map`", so write the code that way instead of "if `s2` is not preferred
over `s1` ...".
This doesn't change any behavior now that `compare_signals()` is a total order,
i.e. `s1` is preferred over `s2`, `s2` is preferred over `s1`, or `s1` and `s2` are equal.
Now, when `s1` and `s2` are equal, we don't call `assign_map.add(s1)`, but that's
already a noop in that case.
7d53d64a47