How to prevent rendering label on lower layer if there will be label on higher one?

[zdila]

I would like to prevent rendering label on lower (L) layer if there will be label on higher one (H). I can't change order of the layers because there is another layer which should be above (L) and below (H).

[talaj]

With current Mapnik you can only render labels from (H) before (L) with opacity="0", then clear collision detector cache before (H) by clear-label-cache="true" to allow to render them again with original opacity. But to clean all the collision cache is probably not desired. It would need something like Keyed collision caches.

[zdila]

Thanks for the answer.