nodes in the tree are nested with respect to one another.
a simple change to the Visitor interface makes it possible
to do this (for example to maintain a current node-depth, or a
knowledge of the name of the current function).

Visit(nil) is called at the end of a node's children;
this make possible the channel-based interface below,
amongst other possibilities.

It is still just as simple to get the original behaviour - just
return the same Visitor from Visit.

Here are a couple of possible Visitor types.

// closure-based
type FVisitor func(n interface{}) FVisitor
func (f FVisitor) Visit(n interface{}) Visitor {
	return f(n);
}

// channel-based
type CVisitor chan Visit;
type Visit struct {
	node interface{};
	reply chan CVisitor;
};
func (v CVisitor) Visit(n interface{}) Visitor
{
	if n == nil {
		close(v);
	} else {
		reply := make(chan CVisitor);
		v <- Visit{n, reply};
		r := <-reply;
		if r == nil {
			return nil;
		}
		return r;
	}
	return nil;
}

R=gri
CC=rsc
https://golang.org/cl/166047