function _acceptAdmin() external returns (uint256) {
    // Check caller is pendingAdmin and pendingAdmin  address(0)
    if (msg.sender != pendingAdmin || msg.sender == address(0)) {
        return fail(Error.UNAUTHORIZED, FailureInfo.ACCEPT_ADMIN_PENDING_ADMIN_CHECK);
    }
    // Save current values for inclusion in log
    address oldAdmin = admin;
    address oldPendingAdmin = pendingAdmin;
    // Store admin with value pendingAdmin
    admin = pendingAdmin;
    // Clear the pending value
    pendingAdmin = address(0);
    emit NewAdmin(oldAdmin, admin);
    emit NewPendingAdmin(oldPendingAdmin, pendingAdmin);
    return uint256(Error.NO_ERROR);
}
function _proposePendingAdmin(address newPendingAdmin) external {
    if (msg.sender != admin) {
        return fail(Error.UNAUTHORIZED, FailureInfo.PROPOSE_PENDING_ADMIN_CHECK);
    }
    address oldPendingAdmin = pendingAdmin;
    pendingAdmin = newPendingAdmin;
    emit NewPendingAdmin(oldPendingAdmin, newPendingAdmin);
    return uint256(Error.NO_ERROR)
}
