public override IResultValue Evaluate()
        {
            IResultValue list  = List.Evaluate();
            IResultValue index = Index.Evaluate();

            if (index.Type != MathType.Integer)
            {
                throw new EvaluationException(this, "Ordinal index must be integer.");
            }
            if (list.Type != MathType.List)
            {
                throw new EvaluationException(this, "Ordinal access can only be used on a list.");
            }

            return(new ResultNumberReal(list.ToList()[(int)index.ToInteger()]));
        }