Reflects selected objects about a user-specified plane (externally defined: geom3d ObjectARX application)
(mirror3d arg1 arg2 ...)
mirror3d function are the same as if you were entering the MIRROR3D AutoCAD command. To signify a user pressing ENTER without typing any values, use nil or an empty string ("").The order, number, and type of arguments for the
T if successfu;, otherwise nil.
The following example mirrors the selected objects about the XY plane that passes through the point 0,0,5, and then deletes the old objects:
(setq ss (ssget))
(mirror3d ss "XY" '(0 0 5) "Y")