code: @Override
      public Set<URI> listOutputs(URI operationUri) {
          if (operationUri == null) {
              return ImmutableSet.of();
          }
  
          return ImmutableSet.copyOf(this.opOutputMap.get(operationUri));
      }
docs: Obtains the list of output URIs for a given Operation
  
  @param operationUri the operation URI
  @return a List of URIs with the outputs of the operation. If no output is provided the List should be empty NOT null.