code: private void set(Unit unit, Rational rational, Unit base, boolean replace) {
      Map<Unit, UnitFactor> map = factors.get(unit);
      if (map == null) {
        map = new EnumMap<>(Unit.class);
        factors.put(unit, map);
      }
      if (replace || !map.containsKey(base)) {
        map.put(base,  new UnitFactor(rational, base));
      }
    }
docs: Set a conversion factor that converts UNIT to BASE. If the replace
  flag is true we always overwrite the mapping.