function approve(
    TaikoData.BlockMetadata calldata _meta,
    TaikoData.Transition calldata _tran,
    TaikoData.TierProof calldata _proof
)
    external
    whenNotPaused
    nonReentrant
    returns (bool approved_)
{
    if (_proof.tier != LibTiers.TIER_GUARDIAN) revert INVALID_PROOF();

    bytes32 hash = keccak256(abi.encode(_meta, _tran));
    approved_ = approve(_meta.id, hash);

    if (approved_) {
        deleteApproval(hash);
        ITaikoL1(resolve("taiko", false)).proveBlock(_meta.id, abi.encode(_meta, _tran, _proof));
    }

    emit GuardianApproval(msg.sender, _meta.id, _tran.blockHash, approved_);
}
if (isTopTier) {
    // A special return value from the top tier prover can signal this
    // contract to return all liveness bond.
    bool returnLivenessBond = blk.livenessBond > 0 && _proof.data.length == 32
        && bytes32(_proof.data) == RETURN_LIVENESS_BOND;

    if (returnLivenessBond) {
        tko.transfer(blk.assignedProver, blk.livenessBond);
        blk.livenessBond = 0;
    }
}
bytes32 hash = keccak256(abi.encode(_meta, _tran, _proof));
