Inner Bevel for abitrary shapes in OpenSCAD

This OpenSCAD module will calculate an inner bevel for an arbitrary part.
You need to specify the height of the z-plane at witch to create the bevel.
Replace the module part() with your own part.

This is based on an ingenious idea from GregFrost (http://www.thingiverse.com/thing:135408). He used it to create an (outer) bevel on a arbitrary shape.
The idea is to minkowski() a small square with the shape creating a slightly larger shape and subtracting the initial shape, creating a thin