gh-38837: Fix bug in covering map of simplicial set with degenerate faces.
There is a bug in the method to compute universal covers of simplicial
sets. It is triggered when a nondegenerate cell
has degenerate faces:
```
sage: RP2 = simplicial_sets.RealProjectiveSpace(2)
sage: S3 = simplicial_sets.Sphere(3)
sage: X = S3.wedge(RP2)
sage: XU = X.universal_cover()
------------------------------------------------------------------------
---
KeyError Traceback (most recent call
last)
Cell In[5], line 1
----> 1 X.universal_cover()
File /usr/local/sage97/src/sage/categories/simplicial_sets.py:581, in
SimplicialSets.Pointed.ParentMethods.universal_cover(self)
557 def universal_cover(self):
558 r"""
559 Return the universal cover of the simplicial set.
560 The fundamental group must be finite in order to ensure that
the
(...)
579 Finitely presented group < | >
580 """
--> 581 return self.universal_cover_map().domain()
File /usr/local/sage97/src/sage/categories/simplicial_sets.py:429, in
SimplicialSets.Pointed.ParentMethods.universal_cover_map(self)
427 return self.identity()
428 G, char = self._universal_cover_dict()
--> 429 return self.covering_map(char)
File /usr/local/sage97/src/sage/categories/simplicial_sets.py:499, in
SimplicialSets.Pointed.ParentMethods.covering_map(self, character)
497 char[s] = char[s.nondegenerate()]
498 else:
--> 499 char[s] = char[self.face(s, d)]
500 if s.is_nondegenerate():
501 for g in G:
KeyError: s_1 s_0 *
```
This PR fixes this.
### :memo: Checklist
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38837
Reported by: miguelmarco
Reviewer(s): John H. Palmieri
