@User: Yes. These are described in the "compose" rules in the magic techfile and they work both ways; "erase poly" will remove the poly but leave diffusion in place of the transistor, while "erase ndiff,pdiff" will erase the diffusion but leave the poly (I noticed that, for obscure reasons, "erase alldiff" doesn't work that way). Note, however, that you can only describe one type of device composed by two base layers. So the default type for ndiff + poly is "nfet". But if you replace "nfet" with "nfetlvt", then try to erase poly, you will be left with a gap in the middle and not diffusion. These behaviors could be corrected with additional paint/erase rules in the "compose" section that I have not put in.