どのようなカスタムストアですか?

import { writable } from 'svelte/store';

function addOrRemoveCartItems() {
    const { subscribe, set, update } = writable(0);

    return {
        subscribe,
        add: () => { update(n => n + 1) },
        // Shopping Cart doesnot contain Negative Items
        remove: () => { update(n => n <= 0 ? 0 : n - 1 ) },
        resetCart: () => { set(0) }
    };
}

export const items = addOrRemoveCartItems();

<script>

    import { items } from "./shoppingcart";

</script>

<main>
    <p> Custom Store Example. </p>

    <h1> Total Cart Items { $items }</h1>

    <button on:click={ items.add }> Add to Cart </button>
    <button on:click={ items.remove }> Remove from Cart </button>
    <button on:click={ items.resetCart }> Remove All Items </button>

</main>

<style>
    main {
        text-align: center;
        padding: 1em;
        max-width: 240px;
        margin: 0 auto;
    }


    @media (min-width: 640px) {
        main {
            max-width: none;
        }
    }
</style>