The previously described implicit grammar G I defines a posterior distribution P (d I |s) over a sentence s via a large, indexed PCFG.
this distribution has the property that, when marginalized, it is equivalent to a posterior distribution P (d|s) over derivations in the correspondingly-weighted all-fragments grammar G. even with an explicit representation of G, we would not be able to tractably compute the parse that maximizes P (t|s) = d∈t P (d|s) = d I ∈t P (d I |s) (Sima'an, 1996).