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+//
+// We've been using io.async() to launch tasks. But there's a
+// stronger variant: io.concurrent().
+//
+// The difference:
+//
+//   io.async():
+//     * The function MAY run on a separate unit of concurrency,
+//       or it may run immediately on the caller (synchronously).
+//     * Never fails — if no concurrency is available, it just
+//       runs the function right away.
+//     * More portable, works with all Io backends.
+//
+//   io.concurrent():
+//     * GUARANTEES a separate unit of concurrency.
+//     * Can fail with error.ConcurrencyUnavailable if resources
+//       are exhausted or the backend doesn't support it.
+//     * Use when you NEED the task to run independently of the
+//       caller.
+//
+// What is a "unit of concurrency"? That depends on the backend!
+// The Threaded backend uses OS threads. But the Evented backends
+// (Uring, Kqueue, Dispatch) use M:N green threads / fibers,
+// which can provide concurrency even on a SINGLE OS thread.
+// Your code doesn't need to know the difference.
+//
+// Because concurrent() can fail, you must handle the error:
+//
+//     var future = try io.concurrent(myFn, .{args});
+//     defer _ = future.cancel(io);
+//     const result = future.await(io);
+//
+// Notice the 'try' — that's the key difference in usage!
+//
+// Fix this program to launch the computation concurrently.
+//
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main(init: std.process.Init) !void {
+    const io = init.io;
+
+    // Launch with a guaranteed separate unit of concurrency.
+    // Which Io method guarantees this?
+    // (Hint: unlike io.async, this one can fail!)
+    var future = try io.???(compute, .{io});
+    defer _ = future.cancel(io);
+
+    // Note: All breaks in this excercise (using sleep)
+    // are only necessary for a deterministic result.
+    io.sleep(std.Io.Duration.fromMilliseconds(100), .awake) catch {};
+
+    print("Main continues...\n", .{});
+
+    // Wait 1 second for the output order.
+    io.sleep(std.Io.Duration.fromMilliseconds(200), .awake) catch {};
+
+    print("Main done waiting.\n", .{});
+
+    const result = future.await(io);
+    print("Result: {}\n", .{result});
+}
+
+fn compute(io: std.Io) u32 {
+    print("Computing concurrently!\n", .{});
+    // Simulate some work.
+    io.sleep(std.Io.Duration.fromMilliseconds(400), .awake) catch return 0;
+    return 123;
+}