After using `x`, we know it is active.
Next we use and activate `y`, which has to pop `Uniq(x)` as they have distinct tags.
Finally, we use `x` again even though it is no longer in the stack, triggering UB.
After using `x`, we know it is active.
Next we use and activate `y`, which has to pop `Uniq(x)` as they have distinct tags.
Finally, we use `x` again even though it is no longer in the stack, triggering UB.