public interface Node {
    

    public int subNodeCount(); //number of branches off this node

    public Node getSubNode (int nodeIndex); //get nth branch

    public boolean isTerminal(); //true if this is a terminal node

    public void setSubNode (int nodeIndex, Node value); // set nth branch
    
    public Node cloneTree(); //produce a clone for crossover/mutation

    public int depth(); // max depth of all branches below
    
    public int functionNodeCount(); // number of function nodes below
    
    public int terminalNodeCount(); // number of terminals below

    public int nodeCount(); // number of all nodes below

    public Node getFunctionNode (int index); // get nth function node from tree below
    
    public Node getTerminalNode (int index); // get nth terminal node from tree below

    public Node getNode (int index); // get nth node from tree below

    public String printPretty(); // string describing this node

    public Object run(); // do the thing...
}


//For Add; some of these might look like this:

    public Double run () {
	//add the result of run() of the two subnodes
	return ((Double)(subNodes[0].run()) + (Double)((subNodes[1].run())));
    }

    public Node cloneTree () {
	// deep clone - IMPORTANT - clone the entire structure, don't copy links
	// otherwise mutation and crossover will mutate the parents and siblings at the same time!
    Node result = new Add();
    for (int i=0; i<subNodes.length; i++) {
	result.setSubNode (i, subNodes[i].cloneTree());
    }
    return result;
    }


   public int functionNodeCount () {
       // how many function nodes below?
	int sum = 1;
	for (int i=0; i<subNodes.length; i++) {
	    sum += subNodes[i].functionNodeCount();
	}
	return sum;
    }


    public int depth () {
	// maximum depth of subnotes, +1
	int result;
	int max = 0;
	for (int i=0; i<subNodes.length; i++) {
	    result = subNodes[i].depth ();
	    if (result > max) max = result;
	}
	return max + 1;
    }

   public String printPretty () {
       // for printing the evolved expression
	return "+";
    }