I was just working out some equations and trying to create a pleasing color map,
and I noticed something that i probably should have known, may have known and
forgotten, and might be helpful to highlight as a pitfall in the docs.

If I have an isosurface or parametric object block,

in the "upper" part, I can use x, y, z, u, and v, and they will be interpreted
as "variables" that are stepped through the range of values that the object
encompasses.

in the "lower" part, if I attempt to use an expression or a function that relies
on x, y, z, u, or v, then at this point POV-Ray interprets these as meaning a
vector quantity, rather than a variable scalar value.

At that point, I also tried the x.x dot notation, and all manner of other
tricks, but to no avail.   I think I finally got something that "worked" (no
error) but the value was static, not variable.

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