(S (S (SBAR (WHADVP (WRB When)) (S (VP (VBG serving) (NP (BASENP (DT a) (NN warrant)) (PP (IN under) (BASENP (JJ normal) (NNS conditions))))))) (COMMA ,) (S (BASENP (NNS police)) (VP (VBP are) (S (VP (TO to) (VP (VP (VB knock) (PP (IN at) (BASENP (DT the) (NN door)))) (CC and) (VP (VB identify) (BASENP (PRP themselves))))))))) (PERIOD .))