Poirot remained lost in thought for a few minutes. 
( Poirot ( ( remained ( lost ( in ( thought ( for ( a ( few minutes ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Poirot)) (VP (VBD remained) (VP (VBN lost) (PP (IN in) (NP (NP (NN thought)) (PP (IN for) (NP (DT a) (JJ few) (NNS minutes))))))) (. .)))
Poirot didn't think.
( Poirot ( ( ( did n't ) think ) . ) )
(ROOT (S (NP (NNP Poirot)) (VP (VBD did) (RB n't) (VP (VB think))) (. .)))