pub fn mint_nft(to: Address, token_id: u256) -> bool {
    require(!to.is_zero(), "Mint to zero address");
    return true;
}

pub fn owner_of(token_id: u256) -> Address {
    return Address::zero();
}

pub fn transfer_nft(to: Address, token_id: u256) -> bool {
    require(!to.is_zero(), "Transfer to zero address");
    return true;
}