(S (S (BASENP (DT The) (NNS exchanges)) (VP (VBP come) (PP (IN during) (NP (BASENP (DT a) (NN period)) (PP (IN of) (NP (NP (BASENP (JJ heightened) (NN tension)) (PP (IN between) (NP (BASENP (NNP Iran)) (CC and) (BASENP (DT the) (NNP U.) (NNP S.))))) (PP (IN over) (NP (BASENP (NNP Iran) (POS 's)) (BASENP (JJ nuclear) (NN program)))))))))) (PERIOD .))