import { onAddHandlerByEvt } from "https://deno.land/x/evt@v2.3.1/lib/Evt.ts";
Can be seen as a protected method that can be optionally be implemented by class extending Evt.
Should only be accessible from within the module. Basically it is for allowing StatefulEvt to execute the callback on attach.
type
WeakMap<NonPostableEvtLike<any>, (handler: Handler<any, any>, handlerTrigger: (opResult: readonly [any]) => PromiseLike<void> | undefined) => void>