code: private ComplexCondition getSleeper(final long identifier) throws InvalidIdentifierException {
      final ComplexCondition call = sleeperMap.get(identifier);
      if (null == call) {
        throw new InvalidIdentifierException(identifier);
      }
      return call;
    }
docs: Get a reference to a sleeper with a specific identifier without removing
  it from the sleeper map.
  @param identifier The unique identifier of the sleeper to be retrieved.
  @return The complex condition object associated with the input identifier.
  @throws InvalidIdentifierException The sleeper map does not contain a call
  with the specified identifier.