def _add_str_to_comment(ds, var, s):
    """Add a string to the `comment` attribute of an array in a Dataset"""
    c_ = ds[var].attrs.get('comment')
    c_ = '' if c_ is None else str(c_)
    if c_.endswith('.'):
        c_ += '  '
    ds[var].attrs['comment'] = c_ + s
    return ds
