any rock pile will do for a set . 
(ROOT (S (NP (DT any) (NN rock) (NN pile)) (VP (MD will) (VP (VB do) (PP (IN for) (NP (DT a) (NN set))))) (. .)))
(ROOT (S (NP (DT ) (NN ) (NN )) (VP (MD ) (VP (VB ) (PP (IN ) (NP (DT ) (NN ))))) (. )))