Add layer-2 example.

contracts/layer2/Layer2.sol

@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.8.0 <0.9.0;
+
+/**
+ * Layer exchange oracle.
+ */
+interface L2
+{
+    /**
+     * Burns the layer-2 coins that should be unlocked on the layer-1 blockchain.
+     *
+     * @param account the account to debit.
+     * @param amount the amount to burn.
+     */
+    function burn(address account, uint256 amount) external;
+
+    /**
+     * Mints the layer-2 coins that have been locked on the layer-1 blockchain.
+     *
+     * @param account the account to credit.
+     * @param amount the amount to mint.
+     */
+    function mint(address account, uint256 amount) external;
+}
+
+/**
+ * Layer-1 contract.
+ */
+contract Layer1
+{
+    /**
+     * Layer-2 oracle.
+     */
+    L2 public l2;
+
+    /**
+     * Creates a new contract.
+     *
+     * @param l2_ the layer-2 exchange oracle.
+     */
+    constructor(L2 l2_)
+    {
+        l2 = l2_;
+    }
+
+    /**
+     * Locks the layer-1 coins and mints the same amount on the layer-2 blockchain.
+     */
+    function lock() external payable
+    {
+        require(address(l2) != address(0), "layer-2 oracle must be set");
+        l2.mint(tx.origin, msg.value);
+    }
+
+    /**
+     * Unlocks the layer-1 coins by burning the same amount on the layer-2 blockchain.
+     *
+     * @param amount the amount to redeem.
+     */
+    function unlock(uint256 amount) external
+    {
+        require(address(l2) != address(0), "layer-2 oracle must be set");
+        l2.burn(tx.origin, amount);
+        payable(tx.origin).transfer(amount);
+    }
+}
+
+/**
+ * Layer-2 contract.
+ */
+contract Layer2 is L2
+{
+    /**
+     * Address balances.
+     */
+    mapping(address => uint256) public balance;
+
+    /**
+     * Contract owner.
+     */
+    address public owner;
+
+    /**
+     * Creates a new contract.
+     *
+     * @param owner_ the contract owner.
+     */
+    constructor(address owner_)
+    {
+        owner = owner_;
+    }
+
+    function burn(address account, uint256 amount) external
+    {
+        require(msg.sender == owner, "must be called by contract owner");
+        require(amount >= balance[account], "insufficient balance");
+        balance[account] -= amount;
+    }
+
+    function mint(address account, uint256 amount) external
+    {
+        require(msg.sender == owner, "must be called by contract owner");
+        balance[account] += amount;
+    }
+
+    /**
+     * Sets the contract owner.
+     *
+     * @param owner_ the new contract owner.
+     */
+    function setOwner(address owner_) external
+    {
+        require(owner == address(0), "contract owner already set");
+        owner = owner_;
+    }
+}

contracts/layer2/Layer2_test.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.8.0 <0.9.0;
+
+import "hardhat/console.sol";
+import "remix_tests.sol";
+import "remix_accounts.sol";
+import "./Layer2.sol";
+
+contract Layer2Test
+{
+    address account = TestsAccounts.getAccount(1);
+    uint256 balance = account.balance;
+
+    Layer2 l2 = new Layer2(msg.sender);
+    Layer1 l1 = new Layer1(l2);
+
+    /// #sender: account-1
+    /// #value: 10000000000000000000
+    function testExchange() public payable
+    {
+        l2.setOwner(address(l1));        
+        l1.lock{value: 5000000000000000000}();
+        Assert.equal(l2.balance(account), 5000000000000000000, "should have correct balance");
+        Assert.equal(address(l1).balance, 5000000000000000000, "should have correct balance");
+
+        l1.unlock(5000000000000000000);
+        Assert.equal(l2.balance(account), 0, "should have correct balance");
+        Assert.equal(address(l1).balance, 0, "should have correct balance");
+    }
+}