    function checkMintSolution(uint256 nonce, bytes32 challenge_digest, bytes32 challenge_number, uint testTarget) public view returns(bool success) {
        bytes32 digest = keccak256(abi.encodePacked(challenge_number, msg.sender, nonce));
        if (uint256(digest) > testTarget) revert();
        return (digest == challenge_digest);
    }