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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def Default() -> 'Blockchain': """ Get the default registered blockchain instance. Returns: obj: Currently set to `neo.Implementations.Blockchains.LevelDB.LevelDBBlockchain`. """

        if Blockchain._instance is None:
            Blockchain._instance = Blockchain()
            Blockchain.GenesisBlock().RebuildMerkleRoot()

        return Blockchain._instance