add: hello_rln

.vscode/settings.json

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+{
+    "solidity.compileUsingRemoteVersion": "v0.8.15+commit.e14f2714"
+}

src/HelloRln.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.15;
+
+import "rln-contract/RlnBase.sol";
+import {Ownable} from "openzeppelin-contracts/contracts/access/Ownable.sol";
+
+contract HelloRln is Ownable, RlnBase {
+    bytes2 public constant start = 0xdead;
+    uint256 private membershipDeposit = 0;
+    uint256 private depth = 20;
+
+    constructor(address _poseidonHasher) Ownable() RlnBase(membershipDeposit, depth, _poseidonHasher, address(0)) {}
+
+    function _validateRegistration(uint256 idCommitment) internal pure override {
+        if (bytes2(bytes32(idCommitment)) != start) revert FailedValidation();
+    }
+
+    function _validateSlash(uint256 idCommitment, address payable receiver, uint256[8] calldata proof)
+        internal
+        view
+        override
+        onlyOwner
+    {}
+}

test/HelloRln.t.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.13;
+
+import "forge-std/Test.sol";
+import "../src/HelloRln.sol";
+import {FailedValidation} from "rln-contract/RlnBase.sol";
+
+contract HelloRlnTest is Test {
+    HelloRln public helloRln;
+
+    function setUp() public {
+        helloRln = new HelloRln(address(0));
+    }
+
+    function test__InvalidRegistration(uint256 idCommitment) public {
+        vm.assume(bytes2(bytes32(idCommitment)) != helloRln.start());
+        vm.expectRevert(FailedValidation.selector);
+        helloRln.register(idCommitment);
+    }
+
+    function test__ValidRegistration() public {
+        bytes32 idCommitment = 0xdead000000000000000000000000000000000000000000000000000000000000;
+        helloRln.register(uint256(idCommitment));
+    }
+}