pub fn list_asset(asset_id: Address, price: u256) -> bool {
    require(!asset_id.is_zero(), "Invalid asset");
    require(price > 0, "Price must be positive");
    return true;
}

pub fn buy_asset(asset_id: Address) -> bool {
    require(!asset_id.is_zero(), "Invalid asset");
    return true;
}

pub fn cancel_listing(asset_id: Address) -> bool {
    require(!asset_id.is_zero(), "Invalid asset");
    return true;
}