code: public DerInputStream toDerInputStream() throws IOException {
          if (tag == tag_Sequence || tag == tag_Set)
              return new DerInputStream(buffer);
          throw new IOException("toDerInputStream rejects tag type " + tag);
      }
docs: For "set" and "sequence" types, this function may be used
  to return a DER stream of the members of the set or sequence.
  This operation is not supported for primitive types such as
  integers or bit strings.