I would therefore suggest an extension specifying an "environment" test as a
better approach. I suspect we can come up with half a dozen things such a test
could operate on.
I like this a lot. I was struggling to articulate why I didn't like what was
being proposed, but this sounds very extensible. I already have a
CSieveRunTimeEnvironment class in my solution and was already wanting to make
more information available to the script.
I also agree that this should be an additional extension, primarily so it
doesn't hold up variables, and secondarily cos I don't think it "needs" to be
part of the base variables spec.