Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/structs/BitMaps.sol

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (utils/structs/BitMaps.sol)
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Library for managing uint256 to bool mapping in a compact and efficient way, providing the keys are sequential.
+ * Largely inspired by Uniswap's https://github.com/Uniswap/merkle-distributor/blob/master/contracts/MerkleDistributor.sol[merkle-distributor].
+ */
+library BitMaps {
+    struct BitMap {
+        mapping(uint256 => uint256) _data;
+    }
+
+    /**
+     * @dev Returns whether the bit at `index` is set.
+     */
+    function get(BitMap storage bitmap, uint256 index) internal view returns (bool) {
+        uint256 bucket = index >> 8;
+        uint256 mask = 1 << (index & 0xff);
+        return bitmap._data[bucket] & mask != 0;
+    }
+
+    /**
+     * @dev Sets the bit at `index` to the boolean `value`.
+     */
+    function setTo(BitMap storage bitmap, uint256 index, bool value) internal {
+        if (value) {
+            set(bitmap, index);
+        } else {
+            unset(bitmap, index);
+        }
+    }
+
+    /**
+     * @dev Sets the bit at `index`.
+     */
+    function set(BitMap storage bitmap, uint256 index) internal {
+        uint256 bucket = index >> 8;
+        uint256 mask = 1 << (index & 0xff);
+        bitmap._data[bucket] |= mask;
+    }
+
+    /**
+     * @dev Unsets the bit at `index`.
+     */
+    function unset(BitMap storage bitmap, uint256 index) internal {
+        uint256 bucket = index >> 8;
+        uint256 mask = 1 << (index & 0xff);
+        bitmap._data[bucket] &= ~mask;
+    }
+}