The guessing experiment provides a basis to estimate human expectation of already mentioned DRs (the number of clicks on the respective NPs in text).
this feature captures the distance l t (d) between the position t and the last occurrence of the candidate DR d. As a distance measure, we use the number of sentences from the last mention and exponentiate this number to make the dependence more extreme; only very recent DRs will receive a noticeable weight: exp(−l t (d)).