sage: G = Graph([(0, 1), (1, 2), (2, 2)], immutable=True, loops=True)
sage: G.matching_polynomial()

x^3 - 3*x

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[2], line 1
----> 1 G.matching_polynomial()

File ~/sage/src/sage/graphs/matchpoly.pyx:244, in sage.graphs.matchpoly.matching_polynomial()
    242 L.sort(key=lambda pair: pair[0])
    243 G = G.relabel(perm={L[i][1]: i for i in range(nverts)}, inplace=False)
--> 244 G.allow_loops(False)
    245 
    246 # Initialization of pol, edges* variables.

File ~/sage/src/sage/graphs/generic_graph.py:3540, in GenericGraph.allow_loops(self, new, check)
   3491 """
   3492 Change whether loops are permitted in the (di)graph.
   3493 
   (...)
   3537     []
   3538 """
   3539 if new is False and check:
-> 3540     self.remove_loops()
   3541 self._backend.loops(new)

File ~/sage/src/sage/graphs/generic_graph.py:13771, in GenericGraph.remove_loops(self, vertices)
  13769 for v in vertices:
  13770     if self.has_edge(v, v):
> 13771         self.delete_multiedge(v, v)

File ~/sage/src/sage/graphs/generic_graph.py:13090, in GenericGraph.delete_multiedge(self, u, v)
  13088         self.delete_edge(u, v, l)
  13089 else:
> 13090     self.delete_edge(u, v)

File ~/sage/src/sage/graphs/generic_graph.py:12817, in GenericGraph.delete_edge(self, u, v, label)
  12814             u, v = u
  12815             label = None
> 12817 self._backend.del_edge(u, v, label, self._directed)

File ~/sage/src/sage/graphs/base/static_sparse_backend.pyx:679, in sage.graphs.base.static_sparse_backend.StaticSparseBackend.del_edge()
    677     raise ValueError("graph is immutable; please change a copy instead (use function copy())")
    678 
--> 679 cpdef del_edge(self, object u, object v, object l, bint directed):
    680     r"""
    681     Delete an edge of the graph. No way.

File ~/sage/src/sage/graphs/base/static_sparse_backend.pyx:692, in sage.graphs.base.static_sparse_backend.StaticSparseBackend.del_edge()
    690         ValueError: graph is immutable; please change a copy instead (use function copy())
    691     """
--> 692     raise ValueError("graph is immutable; please change a copy instead (use function copy())")
    693 
    694 def del_edges(self, edges, directed):

ValueError: graph is immutable; please change a copy instead (use function copy())