code: @Nullable
  	public Either<Throwable, R> get(
  			final K operationKey) throws UnknownOperationKeyException {
  		ResultAccessTracker<R> resultAccessTracker;
  		if ((resultAccessTracker = registeredOperationTriggers.get(operationKey)) == null
  			&& (resultAccessTracker = completedOperations.getIfPresent(operationKey)) == null) {
  			throw new UnknownOperationKeyException(operationKey);
  		}
  
  		return resultAccessTracker.accessOperationResultOrError();
  	}
docs: Returns the operation result or a {@code Throwable} if the {@code CompletableFuture}
  finished, otherwise {@code null}.
  
  @throws UnknownOperationKeyException If the operation is not found, and there is no ongoing
  operation under the provided key.