The WordNet hypernym relations, however, do not form a tree, but a DAG, so whenever there are multiple hypernyms, the uniformly-weighted mean is taken of the probabilities conditioning on each of the hypernyms.
p (Sm, I t~,,mi, @3(Sh)  Note that in the first level of back-off, we no longer condition on the head word, but strictly on its synset, and thereafter on hypernyms of that synset; these models, then, get at the heart of our approach, which is to abstract away from lexical head relations, and move to the more general lexico-semantic relations, here represented by synset relations.