code: protected UnitFactorMap add(Unit unit, String n, Unit base) {
      check(unit);
      check(base);
      if (unit == base) {
        throw new IllegalArgumentException("Attempt to define a unit " + unit + " in terms of itself.");
      }
  
      // Set the factor and its inverse if nothing already exists.
      Rational rational = new Rational(n);
      set(unit, rational, base, true);
      return this;
    }
docs: Set a factor to convert the unit into a given base.