at least during their '70s heyday 
(ROOT (PP (ADVP (IN at) (JJS least)) (IN during) (NP (PRP$ their) (`` ') (NNS 70s) (NN heyday))))
(ROOT (PP (ADVP (IN ) (JJS )) (IN ) (NP (PRP$ ) (`` ) (NNS ) (NN ))))