code: public OperatorSubtaskState putSubtaskStateByOperatorID(
  		@Nonnull OperatorID operatorID,
  		@Nonnull OperatorSubtaskState state) {
  
  		return subtaskStatesByOperatorID.put(operatorID, Preconditions.checkNotNull(state));
  	}
docs: Maps the given operator id to the given subtask state. Returns the subtask state of a previous mapping, if such
  a mapping existed or null otherwise.