code: public static List<VM> requiredVMs(Model mo, JSONObject o, String id) throws JSONConverterException {
          checkKeys(o, id);
          Object x = o.get(id);
          if (!(x instanceof JSONArray)) {
              throw new JSONConverterException("integers expected at key '" + id + "'");
          }
          return vmsFromJSON(mo, (JSONArray) x);
      }
docs: Read an expected list of VMs.
  
  @param mo the associated model to browse
  @param o  the object to parse
  @param id the key in the map that points to the list
  @return the parsed list
  @throws JSONConverterException if the key does not point to a list of VM identifiers