BUG: correct algorithm in function region_diff

When the radius is zero, it means that none of the remaining polytopes
remove anything. We are done at this level, and can save the result.

Make the intersection of hyperplanes in `INDICES` empty.
In that way the algorithm will pop out of this level and move on.

Author: krooken

polytope/polytope.py

@@ -2093,12 +2093,18 @@ def region_diff(poly, reg, abs_tol=ABS_TOL, intersect_tol=ABS_TOL,
                     INDICES = np.hstack([INDICES, beg_mi[level] + M])
                     break
             if R < abs_tol:
-                level -= 1
+                # Since no polytope will remove anything, the current set of
+                # hyperplanes must be in the result
                 res = union(res, Polytope(A[INDICES, :], B[INDICES]), False)
-                INDICES[-1] -= M
+                # None of the remaining polytopes removes anything
+                # Indicate that we are done at this level
+                counter[level] = mi[level]
+                # Add the polytope to the indices to get an empty intersection
+                # This will force the algorithm to pop out of this level and
+                # move on
                 INDICES = np.hstack([
                     INDICES,
-                    beg_mi[level] + counter[level] + M
+                    range(beg_mi[level], beg_mi[level] + mi[level])
                 ])
         else:
             if save: