Compute the expectation:
    <diag(K_{X, X})>_p(X)
        - K_{.,.} :: RBF kernel

    :return: N

def _expectation(p, kern, none1, none2, none3, nghp=None):
    """
    Compute the expectation:
    <diag(K_{X, X})>_p(X)
        - K_{.,.} :: RBF kernel

    :return: N
    """
    return kern.Kdiag(p.mu)