--- exercises/091_async7.zig	2026-04-18 23:30:40.963951835 +0200
+++ answers/091_async7.zig	2026-04-18 23:33:47.313340585 +0200
@@ -49,8 +49,8 @@
     for (0..times) |_| {
         // Acquire the lock before modifying shared state.
         // What Mutex method blocks until the lock is acquired?
-        state.mutex.??? catch return;
-        defer state.mutex.unlock(); // <-- what's missing here?
+        state.mutex.lock(io) catch return;
+        defer state.mutex.unlock(io);
 
         // Sleep to give the other tasks a chance to run in the meantime.
         // We do this here only to make nondeterminism more visible.