public fun foldl (f, acc, l) {
              --  ^-- (1,19)
  case l of
    {}     -> acc
  | x : xs -> foldl (f, f (acc, x), xs)
          -- (5,22)--^  ^-- (5,25)
  esac
}

public fun filter (f, l) {
  case l of
    {}    -> {}
  | h : t -> if f (h) then h : filter (f, t) else filter (f, t) fi
  esac
}