Treat yourself and bill it to Si.
( ( ( ( Treat yourself ) and ) ( ( bill it ) ( to Si ) ) ) . )
(ROOT (S (VP (VP (VB Treat) (NP (PRP yourself))) (CC and) (VP (VB bill) (NP (PRP it)) (PP (TO to) (NP (NNP Si))))) (. .)))
Don't treat yourself, Si has to pay for that. 
( ( ( Do n't ) ( treat yourself ) ) ( , ( Si ( ( has ( to ( pay ( for that ) ) ) ) . ) ) ) )
(ROOT (S (S (VP (VBP Do) (RB n't) (VP (VB treat) (NP (PRP yourself))))) (, ,) (NP (NNP Si)) (VP (VBZ has) (S (VP (TO to) (VP (VB pay) (PP (IN for) (NP (DT that))))))) (. .)))