src/sage/categories/loop_crystals.py

@@ -119,8 +119,11 @@ class LoopCrystals(Category_singleton):
             """
             G = Crystals().parent_class.digraph(self, subset, index_set)
             if have_dot2tex():
-                f = lambda u_v_label: ({"backward": u_v_label[2] == 0})
-                G.set_latex_options(edge_options=f)
+                def eopt(u_v_label):
+                    if u_v_label[2] == 0:
+                        return {"dir": "back"}
+                    return {}
+                G.set_latex_options(edge_options=eopt)
             return G
 
 # TODO: Should we make "regular" an axiom?