             return self._value!
         } else {
             throw TimeoutError(self.description)
+        }
+    }
+}
+
+extension LockedBox where T == UserToKernel {
+    func assertParkedRightNow(file: StaticString = #file, line: UInt = #line) throws {
+        SAL.printIfDebug("\(#function)")
+        let syscall = try self.waitForValue()
+        if case .whenReady = syscall {
+            return
+        } else {
+            XCTFail("unexpected syscall \(syscall)", file: (file), line: line)
         }
     }
 }

        }
        try hookedSelector.assertWakeup(file: (file), line: line)
        try syscallAssertions()
        return try box.takeValue().get()
    }
}

            }
        }
        try hookedSelector.assertWakeup()
        try hookedSelector.userToKernel.assertParkedRightNow() // We need the EventLoop to finish running its tick.
        return try box.takeValue().get()
    }
}
