code: def total_flux(flux, A):
      r
      X = set(np.arange(flux.shape[0]))  # total state space
      A = set(A)
      notA = X.difference(A)
  
      """Extract rows corresponding to A"""
      W = flux.tocsr()
      W = W[list(A), :]
      """Extract columns corresonding to X\A"""
      W = W.tocsc()
      W = W[:, list(notA)]
  
      F = W.sum()
      return F
docs: r"""Compute the total flux between reactant and product.
  
      Parameters
      ----------
      flux : (M, M) scipy.sparse matrix
          Matrix of flux values between pairs of states.
      A : array_like
          List of integer state labels for set A (reactant)
  
      Returns
      -------
      F : float
          The total flux between reactant and product