Evaluates the log-probability of the Markov blanket of
        a stochastic owning a particular index.

def i_logp(self, index):
        """
        Evaluates the log-probability of the Markov blanket of
        a stochastic owning a particular index.
        """
        all_relevant_stochastics = set()
        p, i = self.stochastic_indices[index]
        try:
            return p.logp + logp_of_set(p.extended_children)
        except ZeroProbability:
            return -Inf