anchor.diff:      }
   
       var value = this.value;
  -    var mutableDenominators = denominatorUnits.toList();
  -    for (var numerator in newNumerators) {
  -      removeFirstWhere(mutableDenominators, (denominator) {
  -        var factor = _conversionFactor(numerator, denominator);
  +    var oldNumerators = numeratorUnits.toList();
  +    for (var newNumerator in newNumerators) {
  +      removeFirstWhere(oldNumerators, (oldNumerator) {
  +        var factor = _conversionFactor(newNumerator, oldNumerator);
           if (factor == null) return false;
           value *= factor;
           return true;
  
positive.code:       });
      }
  
      var mutableNumerators = numeratorUnits.toList();
      for (var denominator in newDenominators) {
        removeFirstWhere(mutableNumerators, (numerator) {
          var factor = _conversionFactor(denominator, numerator);
          if (factor == null) return false;
          value *= factor;
          return true;
  
negative.code: 
    Value minus(Value other) {
      if (other is SassNumber) {
        return new SassNumber(_coerceUnits(other, (num1, num2) => num1 - num2));
      }
      if (other is! SassColor) return super.minus(other);
      throw new InternalException('Undefined operation "$this - $other".');
  