code: public WalletAppKit setCheckpoints(InputStream checkpoints) {
          if (this.checkpoints != null)
              Closeables.closeQuietly(checkpoints);
          this.checkpoints = checkNotNull(checkpoints);
          return this;
      }
docs: If set, the file is expected to contain a checkpoints file calculated with BuildCheckpoints. It makes initial
  block sync faster for new users - please refer to the documentation on the bitcoinj website
  (https://bitcoinj.github.io/speeding-up-chain-sync) for further details.