def get_bin_bounds_string(bin_index, bin_bounds, to_str=repr, suffix=""):
    """ retuns, for example: '15 to 20 months' given:
      bin_index: the location in bin_bounds to find the start value
      bin_bounds: a list of bounding values
      suffix: the class of the bounds. EG 'months' or 'days'
    """
    return "{} to {} {}".format(to_str(bin_bounds[bin_index]), to_str(bin_bounds[bin_index + 1]), suffix)