def _em_initial_state_mean(smoothed_state_means):
    """Apply the EM algorithm to parameter `initial_state_mean`

    Maximize expected log likelihood of observations with respect to the
    initial state distribution mean `initial_state_mean`.

    .. math::

        \\mu_0 = \\mathbb{E}[x_0]
    """
    return smoothed_state_means[0]
