It can be easily seen that we must assign probability 1 to all transitions except τ c and τ d , since this is the only pair of distinct transitions that can be applied for one and the same top-of-stack symbol, viz.
whereas in the LR PPDA model we have we conclude that there is no proper assignment of probabilities to the transitions of the LR automaton that would result in a distribution on the generated language that is equivalent to the one induced by the source PCFG.