head.setNext(node); node.setPrevious(head);
    public Trace(TraceNode node) {	if(!(node instanceof FilterTraceNode))	    Utils.fail("FilterTraceNode expected: "+node);	head = new InputTraceNode();	head.setParent(this);	head.setNext(node);	node.setPrevious(head);	depends = new Trace[0];	edges = new Trace[0];	len=-1;	dependsTemp=new ArrayList();	edgesTemp=new ArrayList();    }