Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/crosschain/optimism/LibOptimism.sol

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.7.0) (crosschain/optimism/LibOptimism.sol)
+
+pragma solidity ^0.8.4;
+
+import {ICrossDomainMessenger as Optimism_Bridge} from "../../vendor/optimism/ICrossDomainMessenger.sol";
+import "../errors.sol";
+
+/**
+ * @dev Primitives for cross-chain aware contracts for https://www.optimism.io/[Optimism].
+ * See the https://community.optimism.io/docs/developers/bridge/messaging/#accessing-msg-sender[documentation]
+ * for the functionality used here.
+ */
+library LibOptimism {
+    /**
+     * @dev Returns whether the current function call is the result of a
+     * cross-chain message relayed by `messenger`.
+     */
+    function isCrossChain(address messenger) internal view returns (bool) {
+        return msg.sender == messenger;
+    }
+
+    /**
+     * @dev Returns the address of the sender that triggered the current
+     * cross-chain message through `messenger`.
+     *
+     * NOTE: {isCrossChain} should be checked before trying to recover the
+     * sender, as it will revert with `NotCrossChainCall` if the current
+     * function call is not the result of a cross-chain message.
+     */
+    function crossChainSender(address messenger) internal view returns (address) {
+        if (!isCrossChain(messenger)) revert NotCrossChainCall();
+
+        return Optimism_Bridge(messenger).xDomainMessageSender();
+    }
+}