-- | A list of all continuation labels that only appear once.
usedOnce :: (Ord c) => System c b e a -> [c]
usedOnce sys = uniqueElems $ concatMap continues $
  systemStart sys : Map.elems (systemPaths sys)
usedOnce :: (Ord c) => System c b e a -> [c]
usedOnce sys = uniqueElems $ concatMap continues $
  systemStart sys : Map.elems (systemPaths sys)
usedOnce sys = uniqueElems $ concatMap continues $
  systemStart sys : Map.elems (systemPaths sys)