could have -- should have -- been allowed 
(ROOT (VP (MD could) (VP (VP (VB have)) (, --) (VP (MD should) (VP (VB have))) (, --) (VP (VBN been) (VP (VBN allowed))))))
(ROOT (VP (MD ) (VP (VP (VB )) (, ) (VP (MD ) (VP (VB ))) (, ) (VP (VBN ) (VP (VBN ))))))