How to use the @comunica/bus-rdf-join.ActorRdfJoin.joinVariables function in @comunica/bus-rdf-join

// TODO: This will not work for larger streams.
      // The full inner stream is kept in memory.
      joinedStream.on('end', () => nextLeft());
      joinedStream.on('data', async ({ joinedBindings, result }) => {
        if (result) {
          bindingsStream._push(joinedBindings);
        }
      });
    };

    const transform = leftJoinOuter;
    const bindingsStream = left.bindingsStream
      .transform({ optional: true, transform });

    const variables = ActorRdfJoin.joinVariables({ entries: [left, right] });
    const metadata = () => Promise.all([left, right].map((entry) => entry.metadata()))
      .then((metadatas) => metadatas.reduce((acc, val) => acc * val.totalItems, 1))
      .catch(() => Infinity)
      .then((totalItems) => ({ totalItems }));

    return { type: 'bindings', bindingsStream, metadata, variables };
  }
}

public async getOutput(action: IActionRdfJoin): Promise {
    const variables = ActorRdfJoin.overlappingVariables(action);
    const join = new HashJoin(
      action.entries[0].bindingsStream, action.entries[1].bindingsStream,
      (entry) => ActorRdfJoinHash.hash(entry, variables), ActorRdfJoin.join);
    return { type: 'bindings', bindingsStream: join, variables: ActorRdfJoin.joinVariables(action) };
  }

public async getOutput(action: IActionRdfJoin): Promise {
    const variables = ActorRdfJoin.overlappingVariables(action);
    const join = new SymmetricHashJoin(
      action.entries[0].bindingsStream, action.entries[1].bindingsStream,
      (entry) => ActorRdfJoinSymmetricHash.hash(entry, variables), ActorRdfJoin.join);
    return { type: 'bindings', bindingsStream: join, variables: ActorRdfJoin.joinVariables(action) };
  }

protected async getOutput(action: IActionRdfJoin): Promise {
    const join = new NestedLoopJoin(
      action.entries[0].bindingsStream, action.entries[1].bindingsStream, ActorRdfJoin.join);
    return { type: 'bindings', bindingsStream: join, variables: ActorRdfJoin.joinVariables(action) };
  }