data bt {n : Level} (A : Set n) : Set n where
  leaf : bt A
  node :  (key : ℕ) → (value : A) →
    (left : bt A ) → (right : bt A ) → bt A

record Env {n : Level} (A : Set n) : Set n where
  field
    varn : ℕ
    varv : A
    vart : bt A
    varl : List (bt A)
open Env