could fail to crack a smile at 
(ROOT (S (VP (MD could) (VP (VB fail) (S (VP (TO to) (VP (VB crack) (NP (DT a) (NN smile)) (PP (IN at)))))))))
(ROOT (S (VP (MD ) (VP (VB ) (S (VP (TO ) (VP (VB ) (NP (DT ) (NN )) (PP (IN )))))))))