仮想仔猫ゲーム CryptoKitties のコントラクト解読その２ ref: https://qiita.com/blueplanet/items/e257450834a5a32f0f10

file0.txt

pragma solidity ^0.4.0;

contract Cat {
    function color() public constant returns (string);
}

// エラー内容
This contract does not implement all functions and thus cannot be created.

file1.txt

pragma solidity ^0.4.0;

contract Cat {
    function color() public constant returns (string);
}

contract BlackCat is Cat {
    function color() public constant returns (string) {
        return "Black";
    }
}

file2.txt

...
GeneScienceInterface public geneScience;
function setGeneScienceAddress(address _address) external onlyCEO {
    GeneScienceInterface candidateContract = GeneScienceInterface(_address);

    require(candidateContract.isGeneScience());

    // Set the new contract address
    geneScience = candidateContract;
}

// 仔猫の生成処理の中で、`geneScience.mixGenes(...)`を呼び出している
...