( ( The lens ) ( ( may ( be ( ( convex or ) concave ) ) ) . ) )
(ROOT (S (NP (DT The) (NN lens)) (VP (MD may) (VP (VB be) (ADJP (JJ convex) (CC or) (JJ concave)))) (. .)))
The lens may be convex or concave.
(ROOT (S (NP (JJ Concave) (NNS lenses)) (VP (MD can) (VP (VB correct) (NP (NN myopia)))) (. .)))