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