{-@ LIQUID "--totality" @-}

{-@ predicate NonNull X = ((len X) > 0) @-}

{- risers :: (Ord a) => zs:[a] -> {v: [[a]] | ((NonNull zs) => (NonNull v)) } @-} 
risers []        
  = []
risers []        
  = []
risers []        
  = []