OTHERS CASES THAN CONVEXES POLYHEDRONS
RIDGE BETWEEN A RECTILINEAR EDGE AND A CIRCULAR ONE
RIDGE BETWEEN TWO CIRCULAR EDGES
The ridge is the ELLIPSE with center of the circles being the focus. OM + OM' = R + R' 
The ridge betwen the of two holes is an HYPERBOLA. 
CASE OF A CONCAVE POLYGON
The arrows indiacte the path of the sand .

(P) is the parabola with focus F and directrix (D).
