public abstract class ExtDepthFirstVisitor<T, S> : DepthFirstAstVisitor<T, S>
    {
        protected abstract S PreVisit(AstNode node, T data);
        protected abstract S PostVisit(AstNode node, T data);

        protected override S VisitChildren(AstNode node, T data)
        {
            PreVisit(node, data);

            AstNode next;
            for (var child = node.FirstChild; child != null; child = next)
            {
                // Store next to allow the loop to continue
                // if the visitor removes/replaces child.
                next = child.NextSibling;
                child.AcceptVisitor(this, data);
            }

            PostVisit(node, data);

            return default(S);
        }
    }