/// @title Auction Loan Liquidator
/// @author Florida St
/// @notice NFTs that represent bundles.
contract UserVault is ERC721, ERC721TokenReceiver, IUserVault, Owned {
...
    /// @notice ERC20 balances for a given vault: token => (vaultId => amount). address(0) = ETH
    mapping(address token => mapping(uint256 vaultId => uint256 amount)) _vaultERC20s;
