How to extend Angular Signal and add array slice method?

Need a little help extending an Angular version 19 signal to include an Array slice method while still emitting signal changes. This is the first time I have worked with the Typescript type keyword (or signals for that matter.)

My confusion is how to convey x as an array properly in the signalUpdateFn( node, (x) => {}) function below.
The error is TS2345, Argument of type (x:A) => A[number][] is not assignable to (value:A) => A.

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I'd use my custom signal type in a service or component, something like below, but never got to this point because of next section.

// #products = signal< Product[] >( [] ); // traditional
#myproducts:ExtendedSignal< Product[] > = signal< Product[] >( [] ); // attempt


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Here's my attempt at extending a signal to include a custom method:

import { CreateSignalOptions, signal, Signal, WritableSignal } from "@angular/core";
import { SIGNAL, SignalGetter, signalUpdateFn } from "@angular/core/primitives/signals";

// The & combines (intersects) the "types" of Signal, WritableSignal, and
// { what we specify } together
// <A> is a generic type parameter. I used A to represent an Array
// extends [] I believe means A is constrained to be that object type of Array
// or at least be an object that is not null, undefined, etc.

export type ExtendedSignal<A extends []> = Signal<A> & WritableSignal<A> &
{
// Mimic Array function splice
// splice(start, deleteCount, item1, item2, ...)

splice(value: A, start:number, deleteCount:number, ...items:any): void;
};

export function extendedSignal<A extends []>(
initialValue: A, options?: CreateSignalOptions<A>): ExtendedSignal<A> {

const thissignal = signal<A>(initialValue) as SignalGetter<A> & WritableSignal<A>;
const node = thissignal[SIGNAL];

return Object.assign(thissignal, {
splice: (value: A, start: number, deleteCount: number, ...items: any) =>
signalUpdateFn(node, (x) => { // Error is here
const arraycopy = [...value];
arraycopy.splice(start, deleteCount, ...items);
return arraycopy;
})
});
}


It underlines the (x) => { part with (x:A) => A[number][] cannot be assigned to (value:A) => A.

I am sure I am confusing a few things, but this is how we learn.

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