More of a magic question, but I'm trying to read in SUBCUT from GDS and subtract DNWELL from that. Doesn't work. So maybe magic draws the layers one step at a time? This is what I'm trying.

layer rawdnwell DNWELL

 templayer alldnwell DNWELL
 or lowerdnwell
 copyup lowerdnwell

 layer dnwell DNWELL
 or lowerdnwell
 labels DNWELL

 layer rawisosub SUBCUT

 templayer alliso SUBCUT
 or loweriso
 copyup loweriso

 layer isosub SUBCUT
 or loweriso
 and-not dnwell
 labels SUBCUT

The raw layers are just to save the data for cifout. I want isosub to be cumulative SUBCUT and-not cumulative DNWELL, where the cumulative result includes all the lower hierarchies. But I guess since dnwell and isosub are on the same plane, dnwell gets overwritten by the initial

layer isosub SUBCUT

. I'll try with templayers.