docs: function createService (app, Service, Model, options) {
    Model = options.Model || Model;
    if (typeof Model === 'function') {
      assert(options.ModelName, 'createService but options.ModelName not provided');
      options.Model = Model(app, options.ModelName);
    } else {
      options.Model = Model;
    }
    const service = new Service(options);
    return service;
  }