"Easiest" and fastest thing to do is stop speculating and crank out some
numbers.

Have one shear matrix and another and apply them individually to a VECTOR, and
spit out the result to #debug with vstr.

Then have POV-Ray sequentially apply the matrices and spit out the result.

Apply your matrix transforms to a "matrix" by matrix transforming your basic
vectors x-hat, y-hat, and z-hat and stack the transform output in the #debug
stream to see what matrix you're actually applying in both individual cases and
your compound matrix transform case.

You can use the result vectors to plot spheres in a scene.
Match them up with the corners of the checkerboard pattern.

If anything is mis-matched, it's probably not POV-Ray, but some matrix
construction error.

Run all of the calculations with POV-Ray matrix transforms and vectors to check
and verify your calculations in the above post.

- BW

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