NodeFunctionCall.cs

public interface IContext
{
    double ResolveVariable(string name);
    double CallFunction(string name, double[] arguments);
}

public class NodeFunctionCall : Node
{
    public NodeFunctionCall(string functionName, Node[] arguments)
    {
        _functionName = functionName;
        _arguments = arguments;
    }

    string _functionName;
    Node[] _arguments;

    public override double Eval(IContext ctx)
    {
        // Evaluate all arguments
        var argVals = new double[_arguments.Length];
        for (int i=0; i<_arguments.Length; i++)
        {
            argVals[i] = _arguments[i].Eval(ctx);
        }

        // Call the function
        return ctx.CallFunction(_functionName, argVals);
    }
}