def cull(data, index, min=None, max=None):
    """Sieve an emcee clouds by excluding walkers with search variable 'index'
    smaller than 'min' or larger than 'max'."""
    ret = data
    if min is not None:
        ret = ret[(ret[:, (index)] > min), :]
    if max is not None:
        ret = ret[(ret[:, (index)] < max), :]
    return ret
