( ( 2.2.2 ( Low ( density ( gas ( Neufeld et ) ) ) ) ) ( al. . ) )
(ROOT (S (NP (NNP 2.2.2) (NNP Low) (NN density) (NN gas) (NNP Neufeld) (NNP et)) (VP (VBP al.)) (. .)))
2.2.2 Low density gas Neufeld et al.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))