{-@ LIQUID "--pruneunsorted" @-}

module Foo () where
import Data.Set (Set(..)) 

{-@ measure listKeys :: [(k, v)] -> (Set k)
    listKeys([])   = {v | Set_emp v }
    listKeys(x:xs) = {v | v = Set_cup (Set_sng (fst x)) (listKeys xs) }
  @-}


{-@ getFsts :: ys:[(a, a)] -> {v:[a] | listElts v = listKeys ys } @-}
getFsts ::[(a, a)] ->  [a]
getFsts []           = []
getFsts ((_, x): xs) = x : getFsts xs