docs: public SoyMsgBundle compact(SoyMsgBundle input) {
      ImmutableList.Builder<SoyMsg> builder = ImmutableList.builder();
      for (SoyMsg msg : input) {
        ImmutableList<SoyMsgPart> parts = compactParts(msg.getParts());
        builder.add(
            SoyMsg.builder()
                .setId(msg.getId())
                .setLocaleString(msg.getLocaleString())
                .setIsPlrselMsg(MsgPartUtils.hasPlrselPart(parts))
                .setParts(parts)
                .build());
      }
      return new RenderOnlySoyMsgBundleImpl(input.getLocaleString(), builder.build());
    }