Note that there might be more than one such pair that are compatible with R. For example, for figure 1 step (2), (w 2 , P), (w 3 , NN), (w 4 , NN), (w 5 , AD) and (w 6 , VV) are all compatible with the gold tag sequence.
at the beginning of each step, a compatible set of (t, l) pairs are initialized and  (Goldberg and Elhadad, 2010).