Just getting the hang of things, but I'm finding normals reversed for mirrored boxes (and not spheres) when using fx, fy, or fz as in:
2 * { fx x 4 } box
Expecting two boxes with just "{fx x 4} box" is probably due to my lack of understanding--this instead builds just the mirrored box, with inward-facing normals. (Again, sphere seems O.K.) (Pet peeve: The sensitivity to spaces is a little annoying, at least initially. E.g., "2*" causes a syntax error.)
Combining mirrors as in "2 * { fx fy x 4 } box" builds good boxes, (outward-facing normals).
I'm probably just confused, but I can't seem to get all three mirror planes working:
8 * { fz fy fx x 4 y 4 z 4 } box
...produces just two boxes, and one with normals reversed again....
FWIW!
Matthew