public double Evaluate(IDictionary<string, double> variableList)
{
    var operandQueue = new Queue<double>();

    var indexValidAt = _GetIndexToCalculateTo(_Genome);

    for (var i = indexValidAt; i >= 0; i--)
    {
        var theCharacter = _Genome[i];
        if (!_IsOperator(theCharacter))
            operandQueue.Enqueue(variableList[theCharacter.ToString()]);
        else
        {
            if(_GetNumberOfChildrenFor(theCharacter) > operandQueue.Count)
                return double.NaN;
            switch (theCharacter)
            {
                case '*':
                    operandQueue.Enqueue(operandQueue.Dequeue() * operandQueue.Dequeue());
                    break;
                case '/':
                    operandQueue.Enqueue(operandQueue.Dequeue() / operandQueue.Dequeue());
                    break;
                case '-':
                    operandQueue.Enqueue(operandQueue.Dequeue() - operandQueue.Dequeue());
                    break;
                case '+':
                    operandQueue.Enqueue(operandQueue.Dequeue() + operandQueue.Dequeue());
                    break;
                case 'r':
                    operandQueue.Enqueue(Math.Sqrt(operandQueue.Dequeue()));
                    break;
                default:
                    throw new NotImplementedException("This is NOT an operator!!! '" + theCharacter + "'");
            }

        }

    }

    return operandQueue.Dequeue();
}

static int _GetIndexToCalculateTo(string genome) {
    var genomeLength = genome.Length;
    var indexValidAt = 0;
    var currentScore = 1;
    for (var i = 0; i < genomeLength; i++ )
    {
        var theCharacter = genome[i];
        currentScore += _GetNumberOfChildrenFor(theCharacter) - 1;

        if(currentScore == 0)
        {
            indexValidAt = i;
            break;
        }
    }
    return indexValidAt;
}