//Method for detaching the handler from the Evt, returns false if
//if invoked when the handler is no longer attached.
//The promise returned by the attach*() and waitFor() method.
/* Properties that depends on the method used to attach the handler */
//true if the handler was attached using a method containing "prepend"
//in it's name. Example: evt.$attachOncePrepend(...)
//... if the method contained "extract"
//... if the method contained "once"
//if the method was waitFor()
/* Properties passed as argument to the method used to attach the handler */
//Default: ()=> true, a filter that matches all events.
//Undefined only when the handler was attached using evt.waitFor()
callback?: (transformedData: U)=> void;