warnings/errors on unused definition values

[matdibu]

could we have a warning/error when a value is unused in a writeXML call?

for example: I used a non-existent devices.disk.serial value, only to realize it's not actually supported

[AshleyYakeley]

I think that would add a lot of complication to the nix-to-XML code?

[matdibu]

I can't figure out exactly how xml.toText (process obj) works, could you give me some pointers?