(S (S (PP (IN On) (BASENP (NNP April) (NN 22))) (COMMA ,) (S (BASENP (DT an) (NN oil) (NN slick)) (VP (VBD appeared) (PP (IN on) (NP (BASENP (DT the) (NN surface)) (PP (IN of) (BASENP (DT the) (NNP Gulf)))))))) (PERIOD .))