is no doubt true , but serves as a rather thin moral to such a knowing fable 
(ROOT (S (VP (VBZ is) (ADVP (RB no) (NN doubt)) (ADJP (JJ true))) (, ,) (CC but) (VP (VBZ serves) (PP (IN as) (NP (NP (DT a) (ADJP (RB rather) (JJ thin)) (NN moral)) (PP (IN to) (NP (PDT such) (DT a) (VBG knowing) (NN fable))))))))
(ROOT (S (VP (VBZ ) (ADVP (RB ) (NN )) (ADJP (JJ ))) (, ) (CC ) (VP (VBZ ) (PP (IN ) (NP (NP (DT ) (ADJP (RB ) (JJ )) (NN )) (PP (IN ) (NP (PDT ) (DT ) (VBG ) (NN ))))))))