📚 add generic example

test/invariant/README.md

@@ -0,0 +1,60 @@
+example
+
+```solidity
+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity ^0.8.13;
+
+import {Test} from "forge-std/Test.sol";
+
+contract ExampleTypeHandler is Test {
+    function canResultInOverflow(bytes memory x) public pure {
+        // logic that can result in overflow given x
+    }
+
+    function canResultInUnderflow(bytes memory x) public pure {
+        // logic that can result in underflow given x
+    }
+}
+
+contract ExampleHandler is Test {
+    ExampleTypeHandler public exampleTypeHandler;
+
+    constructor(ExampleTypeHandler _exampleTypeHandler) {
+        exampleTypeHandler = _exampleTypeHandler;
+    }
+
+    function ghost_overflow(bytes memory x) public view {
+        exampleTypeHandler.canResultInOverflow(x);
+    }
+
+    function ghost_underflow(bytes memory x) public view {
+        exampleTypeHandler.canResultInUnderflow(x);
+    }
+}
+
+contract ExampleInvariantOverflow is Test {
+    ExampleHandler public exampleHandler;
+
+    constructor(ExampleHandler _exampleHandler) {
+        exampleHandler = _exampleHandler;
+    }
+
+    function invariant_overflow() public view {
+        bytes memory x;
+        exampleHandler.ghost_overflow(x);
+    }
+}
+
+contract ExampleInvariantUnderflow is Test {
+    ExampleHandler public exampleHandler;
+
+    constructor(ExampleHandler _exampleHandler) {
+        exampleHandler = _exampleHandler;
+    }
+
+    function invariant_underflow() public view {
+        bytes memory x;
+        exampleHandler.ghost_underflow(x);
+    }
+}
+```