code: private List<Unit> getBases(Unit unit) {
      Map<Unit, UnitFactor> map = factors.get(unit);
      if (map != null) {
        return map.values().stream().map(u -> u.unit()).collect(Collectors.toList());
      }
      return Collections.emptyList();
    }
docs: Fetch all of the existing bases for the given unit. A base is a
  conversion factor that was directly added, or the inverse of one that
  was directly added.