code: private static Set<Node> newNodes(Model mo, JSONObject o, String key) throws JSONConverterException {
          checkKeys(o, key);
          Object x = o.get(key);
          if (!(x instanceof JSONArray)) {
              throw new JSONConverterException("array expected at key '" + key + "'");
          }
          Set<Node> s = new HashSet<>(((JSONArray) x).size());
          for (Object i : (JSONArray) x) {
              int id = (Integer) i;
              Node n = mo.newNode(id);
              if (n == null) {
                  throw JSONConverterException.nodeAlreadyDeclared(id);
              }
              s.add(n);
          }
          return s;
      }
docs: Build nodes from a key.
  
  @param mo  the model to build
  @param o   the object that contains the node
  @param key the key associated to the nodes
  @return the resulting set of nodes
  @throws JSONConverterException if at least one of the parsed node already exists