NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars

nltk.parse.RecursiveDescentParser and SteppingRecursiveDescentParser enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU and/or exhaust the Python recursion stack, pinning a process indefinitely ...