Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/crosschain/amb/LibAMB.sol

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.7.0) (crosschain/amb/LibAMB.sol)
+
+pragma solidity ^0.8.4;
+
+import {IAMB as AMB_Bridge} from "../../vendor/amb/IAMB.sol";
+import "../errors.sol";
+
+/**
+ * @dev Primitives for cross-chain aware contracts using the
+ * https://docs.tokenbridge.net/amb-bridge/about-amb-bridge[AMB]
+ * family of bridges.
+ */
+library LibAMB {
+    /**
+     * @dev Returns whether the current function call is the result of a
+     * cross-chain message relayed by `bridge`.
+     */
+    function isCrossChain(address bridge) internal view returns (bool) {
+        return msg.sender == bridge;
+    }
+
+    /**
+     * @dev Returns the address of the sender that triggered the current
+     * cross-chain message through `bridge`.
+     *
+     * 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 bridge) internal view returns (address) {
+        if (!isCrossChain(bridge)) revert NotCrossChainCall();
+        return AMB_Bridge(bridge).messageSender();
+    }
+}