            if (sameTransition) revert L1_ALREADY_PROVED();

            if (isTopTier) {
                // The top tier prover re-proves.
                assert(tier.validityBond == 0);
                assert(ts.validityBond == 0 && ts.contestBond == 0 && ts.contester == address(0));
            ts_.contestBond = 1; // to save gas
