contract Secondary {
    address private _primary;

    /**
     * @dev Emitted when the primary contract changes.
     */
    event PrimaryTransferred(
        address recipient
    );

    /**
     * @dev Sets the primary account to the one that is creating the Secondary contract.
     */
    constructor () internal {
        _primary = msg.sender;
        emit PrimaryTransferred(_primary);
    }

    /**
     * @dev Reverts if called from any account other than the primary.
     */
    modifier onlyPrimary() {
        require(msg.sender == _primary, "Secondary: caller is not the primary account");
        _;
    }

    /**
     * @return the address of the primary.
     */
    function primary() public view returns (address) {
        return _primary;
    }

    /**
     * @dev Transfers contract to a new primary.
     * @param recipient The address of new primary.
     */
    function transferPrimary(address recipient) public onlyPrimary {
        require(recipient != address(0), "Secondary: new primary is the zero address");
        _primary = recipient;
        emit PrimaryTransferred(_primary);
    }
}
/**
 * @dev A Secondary contract can only be used by its primary account (the one that created it).
 */
function primary() public view returns (address) {
    return _primary;
}

/**
 * @return the address of the primary.
 */