code: private void removeSleeper(final long identifier) throws InvalidIdentifierException {
      final ComplexCondition call = sleeperMap.remove(identifier);
      if (null == call) {
        throw new InvalidIdentifierException(identifier);
      }
    }
docs: Remove the specified call from the sleeper map.
  @param identifier The unique identifier of the call to be removed.
  @throws InvalidIdentifierException The sleeper map does not contain a call
  with the specified identifier.