The flat structure described by the Penn Treebank can be seen in this example: (NP (NN lung) (NN cancer) (NNS deaths)) CCGbank (Hockenmaier and Steedman, 2007) is the primary English corpus for Combinatory Categorial Grammar (CCG) (Steedman, 2000) and was created by a semi-automatic conversion from the Penn Treebank.
ccG is a binary branching grammar, and as such, cannot leave NP structure underspecified.