a tendency to sag in certain places 
(ROOT (NP (DT a) (NN tendency) (S (VP (TO to) (VP (VB sag) (PP (IN in) (NP (JJ certain) (NNS places))))))))
(ROOT (NP (DT ) (NN ) (S (VP (TO ) (VP (VB ) (PP (IN ) (NP (JJ ) (NNS ))))))))