Diagonal (interface)/0.99.5a

Diagonal is a retractable set of functions from Model editor's edit menu.

That function applies an axial symmetry along a plan median to the non-selected axis: It emulates the reflection of a mirror that copies half of the object into the other half. On the right example image, diagonal  behaves like a mirror represented by the red line. In both case, the "mirror" plan starts from the coordinate origins of the axis, and stretches at 45° towards the opposite object edge. The transformation enlarges the object limits if needed,  voxels being the maximum size of each side of an object.
 * either to the whole model if nothing is selected
 * or to the selected voxels

See also mirror function.