If the unlexicalized version is also found to be rare, other variations of the production, depending on whether they include the lexical heads and how many non-terminals (one or two) they consider before and after the potential boundary, are examined one after another (see Fisher and Roark [2007] for details).
we compute the maximum likelihood estimates for a primary production (feature) and its other variations, and use those directly as features with/without binarizing the values.