Multi-union gives assertion error

[rjdverbeek]

When I use the multi-union function I get in some situations an assertion error:

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-155-314e2cf39774> in <module>()
----> 1 current_polygon = polygon.SphericalPolygon.multi_union(vp_set)

D:\PyCharmWorkspace\ODC\EdgeCaseAirspaces\spherical_geometry_master\spherical_geometry\polygon.py in multi_union(cls, polygons)
   1159 
   1160         g = graph.Graph(all_polygons)
-> 1161         return g.union()
   1162 
   1163     def intersection(self, other):

D:\PyCharmWorkspace\ODC\EdgeCaseAirspaces\spherical_geometry_master\spherical_geometry\graph.py in union(self)
    412         self._sanity_check("union - remove degenerate edges")
    413         self._remove_3ary_edges()
--> 414         self._sanity_check("union - remove 3ary edges")
    415         self._remove_orphaned_nodes()
    416         self._sanity_check("union - remove orphan nodes", True)

D:\PyCharmWorkspace\ODC\EdgeCaseAirspaces\spherical_geometry_master\spherical_geometry\graph.py in _sanity_check(self, title, node_is_2)
    389                 assert len(node._edges) % 2 == 0
    390             else:
--> 391                 assert len(node._edges) >= 2
    392             for edge in node._edges:
    393                 assert node in edge._nodes

AssertionError: 

I get the error when creating the union of the following three polygons:

[[SingleSphericalPolygon(array([[0.60853841, 0.07537912, 0.78993607],
        [0.58307793, 0.08937607, 0.80748501],
        [0.59261214, 0.12107428, 0.79633653],
        [0.61725749, 0.10959814, 0.77909014],
        [0.61914432, 0.10482613, 0.77824918],
        [0.60853841, 0.07537912, 0.78993607]]), array([0.60075422, 0.09825597, 0.79337263]))],
 [SingleSphericalPolygon(array([[0.59986044, 0.15391752, 0.7851604 ],
        [0.6235809 , 0.14470431, 0.76824965],
        [0.62643114, 0.13854319, 0.76706572],
        [0.61725749, 0.10959814, 0.77909014],
        [0.59261214, 0.12107428, 0.79633653],
        [0.59180216, 0.12300026, 0.79664367],
        [0.59986044, 0.15391752, 0.7851604 ]]), array([0.60969738, 0.12984616, 0.78192652]))],
 [SingleSphericalPolygon(array([[0.64389077, 0.087042  , 0.76015023],
        [0.6323568 , 0.0595102 , 0.77238812],
        [0.60957772, 0.0722364 , 0.78942822],
        [0.60853841, 0.07537912, 0.78993607],
        [0.61914432, 0.10482613, 0.77824918],
        [0.64120761, 0.09430318, 0.76155086],
        [0.64389077, 0.087042  , 0.76015023]]), array([0.62691089, 0.07966165, 0.77500759]))]]

But I don't get the error when I create a multi-union of the polygons 1 and 2, and with polygons 2 and 3.
I also don't get the error when I first create a multi-union of polygons 1 and 2, and then a multi-union of the resulting union of poly 1 + 2 with the polygon 3.

So
1 + 2 + 3 > assertion error
1 + 2 > correct
2 + 3 > correct
(1 + 2) + 3 > correct

When I turn off DEBUG (=False) I get an empty SphericalPolygon.