may have to keep on looking . 
(ROOT (S (VP (MD may) (VP (VB have) (S (VP (TO to) (VP (VB keep) (PRT (IN on)) (S (VP (VBG looking)))))))) (. .)))
(ROOT (S (VP (MD ) (VP (VB ) (S (VP (TO ) (VP (VB ) (PRT (IN )) (S (VP (VBG )))))))) (. )))