docs: public static TransactionWitness redeemP2WPKH(@Nullable TransactionSignature signature, ECKey pubKey) {
          checkArgument(pubKey.isCompressed(), "only compressed keys allowed");
          TransactionWitness witness = new TransactionWitness(2);
          witness.setPush(0, signature != null ? signature.encodeToBitcoin() : new byte[0]); // signature
          witness.setPush(1, pubKey.getPubKey()); // pubkey
          return witness;
      }