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> >
> > an infinitely long, straight, constant-diameter piece of
> spaghetti
> This can be constructed as the product of three symmetries:
> a1) rotational symmetry around the z-axis
> a2) reflection symmetry in any plane perpendicular to the z axis
> a3) translational invariance along the z-axis
>
Doesn't a2 (particularly with the word *any* in it) imply a3?
I'm not a strict adherent to John's statement as the words above are nice
and short and descriptive and don't involve arcane crystallographic
nomenclature.