chore: add exercise solidity files

program-analysis/echidna/exercises/exerciseX/NaughtCoin.sol

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import {ERC20} from "./level-utils/ERC20.sol";
+
+contract NaughtCoin is ERC20 {
+    // string public constant name = 'NaughtCoin';
+    // string public constant symbol = '0x0' -> 'NTC';
+    // uint public constant decimals = 18;
+    uint256 public timeLock = block.timestamp + 10 * 365 days;
+    uint256 public INITIAL_SUPPLY;
+    address public player;
+
+    // Note that the original symbol is changed from 0x0 to NTC
+    // This is caused by https://github.com/crytic/echidna/issues/909
+    constructor(address _player) ERC20("NaughtCoin", "NTC") {
+        player = _player;
+        INITIAL_SUPPLY = 1000000 * (10**uint256(decimals()));
+        // _totalSupply = INITIAL_SUPPLY;
+        // _balances[player] = INITIAL_SUPPLY;
+        _mint(player, INITIAL_SUPPLY);
+        emit Transfer(address(0), player, INITIAL_SUPPLY);
+    }
+
+    function transfer(address _to, uint256 _value)
+        public
+        override
+        lockTokens
+        returns (bool)
+    {
+        super.transfer(_to, _value);
+    }
+
+    // Prevent the initial owner from transferring tokens until the timelock has passed
+    modifier lockTokens() {
+        if (msg.sender == player) {
+            require(block.timestamp > timeLock);
+            _;
+        } else {
+            _;
+        }
+    }
+}

program-analysis/echidna/exercises/exerciseX/level-utils/Context.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (utils/Context.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Provides information about the current execution context, including the
+ * sender of the transaction and its data. While these are generally available
+ * via msg.sender and msg.data, they should not be accessed in such a direct
+ * manner, since when dealing with meta-transactions the account sending and
+ * paying for execution may not be the actual sender (as far as an application
+ * is concerned).
+ *
+ * This contract is only required for intermediate, library-like contracts.
+ */
+abstract contract Context {
+    function _msgSender() internal view virtual returns (address) {
+        return msg.sender;
+    }
+
+    function _msgData() internal view virtual returns (bytes calldata) {
+        return msg.data;
+    }
+}