https://netbasal.com/creating-custom-operators-in-rxjs-32f052d69457
https://angular.io/guide/observables
https://medium.com/@benlesh/hot-vs-cold-observables-f8094ed53339
https://ncjamieson.com/understanding-publish-and-share/ https://ncjamieson.com/multicasts-secret/
https://www.learnrxjs.io/learn-rxjs/operators/error_handling/retrywhen https://blog.angular-university.io/rxjs-error-handling/