/** @return The filter serialized using pb */
 public byte[] toByteArray() {
   FilterProtos.MultipleColumnPrefixFilter.Builder builder =
       FilterProtos.MultipleColumnPrefixFilter.newBuilder();
   for (byte[] element : sortedPrefixes) {
     if (element != null) builder.addSortedPrefixes(ByteStringer.wrap(element));
   }
   return builder.build().toByteArray();
 }