unfortunately, I recognized that it is not possible to use variables within the carve SOP node. What I want to do is using an expression for "First U" and "First V" within the carve node, depending on custom primitive attributes. It is neither possible to use custom primitve attributes, nor the standard $PR variable within the carve node parameters. But as far as I know, the carve node itself runs in a per-primitive context.
The only way to access primitive attributes in conjuction with the carve node is to build a foreach construct...but this is very slow.
It would be great if anybody knows another, faster solution.
Thanks a lot