private static void printNode(Bdd u)
{
XmlNode node = doc.CreateElement("node");
XmlElement uNode = (XmlElement)doc.CreateNode("element", "u", "");
uNode.InnerText = u.ToString();
XmlElement lowNode = (XmlElement)doc.CreateNode("element", "low", "");
lowNode.InnerText = u.Low.ToString();
XmlElement highNode = (XmlElement)doc.CreateNode("element", "high", "");
highNode.InnerText = u.High.ToString();
XmlElement levelNode = (XmlElement)doc.CreateNode("element", "level", "");
levelNode.InnerText = u.Var.ToString();
node.AppendChild(uNode);
node.AppendChild(lowNode);
node.AppendChild(highNode);
node.AppendChild(levelNode);
doc.ChildNodes.Item(0).AppendChild(node);
}