def _parse_present_bias_parameter(optim_paras, params):
    """Parse present-bias parameter which is 1 by default.

    Examples
    --------
    An example where present-bias parameter is specified:

    >>> tuples = [("beta", "beta")]
    >>> index = pd.MultiIndex.from_tuples(tuples, names=["category", "name"])
    >>> params = pd.Series(data=0.4, index=index)
    >>> optim_paras = {"delta": 0.95}
    >>> _parse_present_bias_parameter(optim_paras, params)
    {'delta': 0.95, 'beta': 0.4, 'beta_delta': 0.38}

    And one where present-bias parameter is not specified:

    >>> params = pd.Series(dtype="float64")
    >>> optim_paras = {"delta": 0.95}
    >>> _parse_present_bias_parameter(optim_paras, params)
    {'delta': 0.95, 'beta': 1, 'beta_delta': 0.95}

    """
    beta = params.get(("beta", "beta"), 1)

    optim_paras["beta"] = beta
    optim_paras["beta_delta"] = beta * optim_paras["delta"]

    return optim_paras