def z_link_func(x, data=mydata):
    stim = (np.asarray(dmatrix('0 + C(s, [[0], [1]])',
                               {'s': data.stimulus.loc[x.index]}))
    )
    # Apply z = (1 - x) to flip them along 0.5
    z_flip = stim - x
    # The above inverts those values we do not want to flip,
    # so invert them back
    z_flip[stim == 0] *= -1
    return z_flip