code: function() {
          const flow = this._chooseFlow();
          console.log("Active flow => %s", JSON.stringify(flow));
          const nextStage = this._firstUncompletedStage(flow);
          console.log("Next stage: %s", nextStage);
          return nextStage;
      }
docs: Pick the next auth stage
  
  @private
  @return {string?} login type
  @throws {NoAuthFlowFoundError} If no suitable authentication flow can be found