def prob_t(self, seq_p, seq_ch, t, pattern_multiplicity = None,
               return_log=False, ignore_gaps=True):
        """
        Compute the probability to observe seq_ch (child sequence) after time t starting from seq_p
        (parent sequence).

        Parameters
        ----------

         seq_p : character array
            Parent sequence

         seq_c : character array
            Child sequence

         t : double
            Time (branch len) separating the profiles.

         pattern_multiplicity : numpy array
            If sequences are reduced by combining identical alignment patterns,
            these multplicities need to be accounted for when counting the number
            of mutations across a branch. If None, all pattern are assumed to
            occur exactly once.

         return_log : bool
            It True, return log-probability.

        Returns
        -------
         prob : np.array
            Resulting probability

        """
        if t<0:
            logP = -ttconf.BIG_NUMBER
        else:
            tmp_eQT = self.expQt(t)
            bad_indices=(tmp_eQT==0)
            logQt = np.log(tmp_eQT + ttconf.TINY_NUMBER*(bad_indices))
            logQt[np.isnan(logQt) | np.isinf(logQt) | bad_indices] = -ttconf.BIG_NUMBER
            seq_indices_c = np.zeros(len(seq_ch), dtype=int)
            seq_indices_p = np.zeros(len(seq_p), dtype=int)
            for ai, a in self.alphabet:
                seq_indices_p[seq_p==a] = ai
                seq_indices_c[seq_ch==a] = ai

            if len(logQt.shape)==2:
                logP = np.sum(logQt[seq_indices_p, seq_indices_c]*pattern_multiplicity)
            else:
                logP = np.sum(logQt[seq_indices_p, seq_indices_c, np.arange(len(seq_c))]*pattern_multiplicity)

        return logP if return_log else np.exp(logP)
Compute the probability to observe seq_ch (child sequence) after time t starting from seq_p
        (parent sequence).

        Parameters
        ----------

         seq_p : character array
            Parent sequence

         seq_c : character array
            Child sequence

         t : double
            Time (branch len) separating the profiles.

         pattern_multiplicity : numpy array
            If sequences are reduced by combining identical alignment patterns,
            these multplicities need to be accounted for when counting the number
            of mutations across a branch. If None, all pattern are assumed to
            occur exactly once.

         return_log : bool
            It True, return log-probability.

        Returns
        -------
         prob : np.array
            Resulting probability