docs: def vert_connectivity(self):
          """Returns a sparse matrix (of size #verts x #verts) where each nonzero
          element indicates a neighborhood relation. For example, if there is a
          nonzero element in position (15, 12), that means vertex 15 is connected
          by an edge to vertex 12."""
          import numpy as np
          import scipy.sparse as sp
          from blmath.numerics.matlab import row
          vpv = sp.csc_matrix((len(self.v), len(self.v)))
          # for each column in the faces...
          for i in range(3):
              IS = self.f[:, i]
              JS = self.f[:, (i+1)%3]
              data = np.ones(len(IS))
              ij = np.vstack((row(IS.flatten()), row(JS.flatten())))
              mtx = sp.csc_matrix((data, ij), shape=vpv.shape)
              vpv = vpv + mtx + mtx.T
          return vpv