(S (S (NP (BASENP (DT The) (NN source)) (PP (IN of) (BASENP (DT the) (NN problem)))) (VP (VBD had) (VP (VBN been) (VP (VBN traced) (ADVP (RB back) (PP (TO to) (NP (BASENP (NN software) (NN maintenance)) (VP (VBN performed) (PP (IN on) (BASENP (NNP March) (NN 4))))))))))) (PERIOD .))