code: public static Node requiredNode(Model mo, JSONObject o, String id) throws JSONConverterException {
          checkKeys(o, id);
          try {
              return getNode(mo, (Integer) o.get(id));
          } catch (ClassCastException e) {
              throw new JSONConverterException("Unable to read a Node identifier from string at key '" + id + "'", e);
          }
      }
docs: Read an expected node.
  
  @param mo the associated model to browse
  @param o  the object to parse
  @param id the key in the map that points to the node identifier
  @return the node
  @throws JSONConverterException if the key does not point to a node identifier