US /ɪnˈveriənt/
・UK /ɪnˈveəriənt/
Now, to prove this, we're going to use what's called an invariant.
In order to show that your system can never reach a particular special state, it is sufficient to show there's some property called the invariant that holds at the initial state and that is preserved by every legal move and is not present, does not hold in that special state.