(S (S (ADVP (RB Theoretically)) (S (NP (BASENP (DT a) (NN craft)) (VP (VBN propelled) (PP (IN by) (NP (BASENP (DT the) (NN pressure)) (PP (IN of) (NP (BASENP (DT a) (JJ constant) (NN light) (NN source)) (PRN (COMMA ,) (PP (JJ such) (PP (IN as) (BASENP (DT a) (JJ purpose-built) (NN laser)))) (COMMA ,)))))))) (VP (MD could) (VP (VB achieve) (NP (BASENP (NNS speeds)) (VP (VBG approaching) (NP (BASENP (DT the) (NN speed)) (PP (IN of) (BASENP (NN light)))))))))) (PERIOD .))