// Free bread while stock lasts. Stock is a per-machine variable that is saved
// with the world, so every machine keeps its own count.
pub fn dispense(ctx) {
    let stock = ctx.get_var("stock");
    if stock <= 0 {
        ctx.message("The vending machine is empty.");
        return false;
    }
    ctx.set_var("stock", stock - 1);
    ctx.give_item("bread", 1);
    ctx.message(`The machine dispenses some bread. (${stock - 1} left)`);
}