<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def total_flux(flux, A):
 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 """

    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