The expression can be expanded further: The reason for this manipulation is that the expected recall score for π is now written in terms of a sum over the individual dependencies in π, rather than a sum over each dependency structure for S. The inner sum is over all derivations which contain a particular individual dependency τ.
the final score for a dependency structure π is a sum of the scores for each dependency τ in π; and the score for a dependency τ is the sum of the probabilities of those derivations producing τ.