code: def _t_update_A(self):
          r
          network = self.project.network
          Vi = network['pore.volume']
          dt = self.settings['t_step']
          s = self.settings['t_scheme']
          if (s == 'implicit'):
              f1, f2 = 1, 1
          elif (s == 'cranknicolson'):
              f1, f2 = 0.5, 1
          elif (s == 'steady'):
              f1, f2 = 1, 0
          # Compute A (operations involve conversion to 'csr')
          A = ((f2/dt) * sprs.coo_matrix.multiply(
              sprs.coo_matrix(np.reshape(Vi, (self.Np, 1)), shape=(self.Np,)),
              sprs.identity(self.Np, format='coo')) + f1 * self._A_steady)
          # Convert A to 'coo' format to apply BCs
          A = sprs.coo_matrix(A)
          self._A = A
          return A
docs: r"""
          A method to update 'A' matrix at each time step according to 't_scheme'