Math.permutations and Math.combinations: Return nil if n or k is negative
Math.permutations and Math.combinations: Return nil if n or k is negative

nicolasmccurdy/magician-clojure,nicolasmccurdy/magician-haskell,nicolasmccurdy/magician-lisp,nicolasmccurdy/magician-clojure,nicolasmccurdy/magician,nicolasmccurdy/magician-lisp