NLTK: Quadratic-time CPU denial of service in Tree.chomsky_normal_form right-factor binarization via list.pop(0)

Summary Tree.chomskynormalform binarizes a parse tree and defaults to right factoring. For a node with N children the right-factor branch repeatedly removes the first element of a working list with list.pop0, which shifts all remaining elements each time, so binarizing one wide node costs ON^2. A...