have banged their brains into the ground so frequently 
(ROOT (S (VP (VBP have) (VP (VBN banged) (NP (PRP$ their) (NNS brains)) (PP (IN into) (NP (DT the) (NN ground))) (ADVP (RB so) (RB frequently))))))
(ROOT (S (VP (VBP ) (VP (VBN ) (NP (PRP$ ) (NNS )) (PP (IN ) (NP (DT ) (NN ))) (ADVP (RB ) (RB ))))))