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contract proofOfExistence {
    mapping (bytes32 => Document) public documents;

    struct Document {
        uint date;
        address creator;
        bytes32 hash;
        address[] signers;
    }
    
    function createNewHash(string document) {
        bytes32 hash = sha3(document);
        address[] signer;
        signer[0] = msg.sender;
        
        if (documents[hash].date > 0) throw;
        
        documents[hash] = Document({ 
            date: now, 
            creator: msg.sender, 
            hash: hash, 
            signers: signer 
        });
    }
    
    function acknowledgeDocument(string document){
        bytes32 hash = sha3(document);
        Document doc = documents[hash];
        uint signerID = doc.signers.length++;
        doc.signers[signerID] = msg.sender;
    }
}