The backing off steps are shown in Figure 1.
if we use the similarity-based language model shown in (2) as a guide, then we can create a smoothed version of Collins' model using the weighted probability of all similar PPs (for brevity we use c in to indicate the context, in this case an entire PP quadruple): to the language model shown in (2), the set of similar contexts S(c) and similarity function α(c, c ) must be defined for multiple words (we abuse our notation slightly by using the same α and S for both PPs and words, but the meaning should be clear from the context).