code: private void addSleeper(final long identifier, final ComplexCondition call) {
      if (sleeperMap.put(identifier, call) != null) {
        throw new RuntimeException(String.format("Duplicate identifier [%d] in sleeper map", identifier));
      }
    }
docs: Atomically add a coll to the sleeper map.
  @param identifier The unique call identifier.
  @param call The call object to be added to the sleeper map.