def _phi(speciation_initiation_rate,
       speciation_completion_rate,
       incipient_species_extinction_rate):
    """
    Returns value of $\varphi$, as given in eq. 6 in Etienne et al.
    (2014).

    Parameters
    ----------

    speciation_initiation_rate : float
        The birth rate, b (the incipient species birth
        rate and the "good" species birth rate are assumed to be equal):
        the rate at which new (incipient) species are produced from
        either incipient or "good" species lineages.
    speciation_completion_rate : float
        The rate at which incipient species get converted to good
        species, $\lambda_1$.
    incipient_species_extinction_rate : float
        The incipient species exctinction rate, $\mu_1$: the rate at which
        incipient species go extinct.

    Returns
    -------
    t : float
        The duration of speciation.

    """
    phi = speciation_completion_rate - speciation_initiation_rate + incipient_species_extinction_rate
    return phi