@Override
 public DigestValue computeDigestValue() throws BigDBException {
   Object leafObject = getObject();
   assert leafObject != null;
   return DigestValue.fromString(
       String.format(
           "%s|%s|%s", getNodeType().name(), getLeafType().name(), leafObject.toString()));
 }
 @Override
 public DigestValue computeDigestValue() throws BigDBException {
   return DigestValue.fromString(getNodeType().name());
 }