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This is the identity function with special type annotations.
If you have a variable that is either an Evt that post A or an Evt that post B you have an event that post A or B.
In other words Evt<A> | Evt<B> is assignable to Evt<A | B >. This method implements this property.
import { Evt, VoidEvt, matchVoid } from "evt";
declare evt: Evt<string> | Evt<number> | VoidEvt = Evt.create<any>();
evt.attach(data=> { }); // TS ERROR
Evt.factorize(evt) // OK, return Evt<string | number | void>
.attach(data=> { // data is string | number | void
//To test if data is void
if( matchVoid(data) ){
//Here data is string | number.
See also FactorizeEvt<E>, helper type that this method levrage.