@User: No, the "unknown layer" error is just that the GDS layer 236:0 is in all the SkyWater files but isn't documented and nobody seems to know what it is supposed to be for. I should probably just ignore it during GDS input, although I have left the error message as a reminder that I'd like to get a definitive answer about it.

Just call it UNKNOWN_FOUNDRY_FLAG1 I guess? might even be the layer for "this is a foundry provided cell"

Thanks for confirming