function [ll, p] = prob_path(prior, transmat, obsmat, qs)
% PROB_PATH Compute the prob. of a specific path (state sequence) through an HMM.
% [ll, p] = prob_path(prior, transmat, obsmat, states)
%
% ll = log prob path
% p(t) = Pr(O(t)) * Pr(Q(t) -> Q(t+1)) for 1<=t