 The term "forward–backward algorithm" is also used to refer to any algorithm belonging to the general class of algorithms that operate on sequence models in a forward–backward manner.In this sense, the descriptions in the remainder of this article refer but to one specific instance of this class.In the first pass, the forward–backward algorithm computes a set of forward probabilities which provide, for all formula_4, the probability of ending up in any particular state given the first formula_5 observations in the sequence, i.e.