(S (S (SBAR (WHADVP (WRB When)) (S (BASENP (NNS rioters)) (VP (VBD tried) (S (VP (TO to) (VP (VB enlarge) (BASENP (DT the) (NN protest)))))))) (COMMA ,) (S (BASENP (NNS police)) (VP (VP (VBD used) (BASENP (NN force))) (S (VP (TO to) (VP (VB stop) (BASENP (PRP them)))))))) (PERIOD .))