For any subsequent SHIFT action (SHIFT, c) to be valid, the necessary condition is c ≡ c lex 0 , where c lex 0 denotes the gold-standard lexical category of the front word in the queue, q 0 (line 3).
this condition is not sufficient; a counterexample is the case where all the goldstandard lexical categories for the sentence in Figure 2 are shifted in succession.