/** Unwind the trail to given low water mark */
public void unwindTrail(int mark) {
for (int i = trail.size() - 1; i >= mark; i--) {
Node_RuleVariable var = (Node_RuleVariable) trail.get(i);
var.unbind();
trail.remove(i);
}
}
/**
* Bind a value to a variable, recording the binding in the trail.
*
* @param var the dereferenced variable to be bound
* @param val the value to bind to it
*/
public void bind(Node var, Node val) {
((Node_RuleVariable) var).simpleBind(val);
trail.add(var);
}