def METHOD_NAME(dls, shape, pmc):
    """Backward derivative in y."""
    Nx, Ny = shape
    if Ny == 1:
        return sp.csr_matrix((Nx, Nx))
    dyb = sp.csr_matrix(sp.diags([1, -1], [0, -1], shape=(Ny, Ny)))
    if pmc:
        dyb[0, 0] = 2.0
    else:
        dyb[0, 0] = 0.0
    dyb = sp.diags(1 / dls).dot(dyb)
    dyb = sp.kron(sp.eye(Nx), dyb)
    return dyb