As defined in (Collins and Duffy, 2002), a syntactic tree fragment f r i is active in t i when f r i is a subtree of the syntactic interpretation of t i .
these features represent ground rules connecting the left-hand sides and the right-hand sides of the tweet pair: each feature is active for a pair (t 1 , t 2 ) when the left-hand side f r 1 is activated by the syntactic analysis of t 1 and the right-hand side f r 2 is activated by t 2 .