docs: private int ascertainParentFingerprint(DeterministicKey parentKey, int parentFingerprint)
      throws IllegalArgumentException {
          if (parentFingerprint != 0) {
              if (parent != null)
                  checkArgument(parent.getFingerprint() == parentFingerprint,
                                "parent fingerprint mismatch",
                                Integer.toHexString(parent.getFingerprint()), Integer.toHexString(parentFingerprint));
              return parentFingerprint;
          } else return 0;
      }