(S (S (BASENP (DT The) (NN border)) (VP (VBZ has) (VP (VBN been) (VP (VP (VBN maintained) (PP (IN by) (BASENP (JJ Russian) (NNS forces)))) (PP (IN since) (BASENP (DT the) (NNS 1990s))))))) (PERIOD .))