code: def Default() -> 'Blockchain':
          
          if Blockchain._instance is None:
              Blockchain._instance = Blockchain()
              Blockchain.GenesisBlock().RebuildMerkleRoot()
  
          return Blockchain._instance
docs: Get the default registered blockchain instance.
  
          Returns:
              obj: Currently set to `neo.Implementations.Blockchains.LevelDB.LevelDBBlockchain`.