NodeSet set = ((ArraySet)context).getRange(doc, f_gid, e_gid);
NodeSet set = contextSet.getRange(doc, f_gid, e_gid);
   public Value eval(DocumentSet docs, NodeSet context, NodeProxy node) {      DocumentImpl doc = node.getDoc();      int level = doc.getTreeLevel(node.getGID());      long pid = (node.getGID() - doc.getLevelStartPoint(level)) /        doc.getTreeLevelOrder(level)        + doc.getLevelStartPoint(level - 1);      long f_gid = (pid - doc.getLevelStartPoint(level -1)) *        doc.getTreeLevelOrder(level) +        doc.getLevelStartPoint(level);      long e_gid = f_gid + doc.getTreeLevelOrder(level);      NodeSet set = ((ArraySet)context).getRange(doc, f_gid, e_gid);      int len = set.getLength();      return new ValueNumber((double)len);   }