I wrote: << Imagine the intersection of N with the unit sphere S^2 about the origin. That will be a number of great circles of S^2.

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This isn't right; rather it's just each 2D cylinder that intersects the unit sphere in a great circle. Now look at how each of these great circles on S^2 is cut by the others; each resulting arc corresponds to one "face" of the boundary of the intersection solid. I hope that was clearer and truer. --Dan _____________________________________________________________________ "It don't mean a thing if it ain't got that certain je ne sais quoi." --Peter Schickele