Where have I seen a variable or POV-Ray function that gives the height of a height field at a given set of x, z coordinates?

If this were a new feature of POV 3.1, it would be the last straw, and I'd have to download it. But I didn't see it in any list of new features, and it's not on my 3.02 documentation. Did I hear about this in one of the specialty versions of POV, or is it my imagination?

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Greg M. Johnson