( ( For example ) ( , ( ( carbon atoms ) ( always ( ( ( ( ( have ( six protons ) ) , ) but ) ( may ( have ( ( ( ( ( six , ) seven ) , ) or ) ( eight neutrons ) ) ) ) ) . ) ) ) ) )
(ROOT (S (PP (IN For) (NP (NN example))) (, ,) (NP (NN carbon) (NNS atoms)) (ADVP (RB always)) (VP (VP (VBP have) (NP (CD six) (NNS protons))) (, ,) (CC but) (VP (MD may) (VP (VB have) (NP (NP (CD six)) (, ,) (NP (CD seven)) (, ,) (CC or) (NP (CD eight) (NNS neutrons)))))) (. .)))
For example, carbon atoms always have six protons, but may have six, seven, or eight neutrons.
(ROOT (S (NP (DT All) (NN carbon) (NNS atoms)) (VP (VBP have) (NP (CD six) (NNS protons))) (. .)))