docs: def vertices_per_edge(self):
          """Returns an Ex2 array of adjacencies between vertices, where
          each element in the array is a vertex index. Each edge is included
          only once. Edges that are not shared by 2 faces are not included."""
          import numpy as np
          return np.asarray([vertices_in_common(e[0], e[1]) for e in self.f[self.faces_per_edge]])