could have -- should have -- been allowed to stand on their own 
(ROOT (VP (MD could) (VP (VB have) (, --) (VP (MD should) (VP (VB have))) (, --) (VP (VBN been) (VP (VBN allowed) (S (VP (TO to) (VP (VB stand) (PP (IN on) (NP (PRP$ their) (JJ own)))))))))))
(ROOT (VP (MD ) (VP (VB ) (, ) (VP (MD ) (VP (VB ))) (, ) (VP (VBN ) (VP (VBN ) (S (VP (TO ) (VP (VB ) (PP (IN ) (NP (PRP$ ) (JJ )))))))))))