code: def nnz(self):
          
          nnz = 0
          for k in range(len(self.snpost)):
              nn = self.snptr[k+1]-self.snptr[k]    
              na = self.relptr[k+1]-self.relptr[k]
              nnz += nn*(nn+1)/2 + nn*na
          return nnz
docs: Returns the number of lower-triangular nonzeros.