{-# OPTIONS --show-implicit #-}
{-# OPTIONS --sized-types #-}
{-# OPTIONS --termination-depth=2 #-}
-- {-# OPTIONS -v term:10 #-}

module SizedBTree where

open import Common.Size

data BTree (A : Set) : {size : Size} → Set where
  leaf : {i : Size} → A → BTree A {↑ i}
  node : {i : Size} → BTree A {i} → BTree A {i} → BTree A {↑ i}

map : ∀ {A B i} → (A → B) → BTree A {i} → BTree B {i}
map f (leaf a) = leaf (f a)
map f (node l r) = node (map f l) (map f r)

-- deep matching

deep : ∀ {i A} → BTree A {i} → A
deep (leaf a) = a
deep (node (leaf _) r) = deep r
deep (node (node l r) _) = deep (node l r)

-- nesting

deep2 : ∀ {i A} → BTree A {i} → BTree A {i}
deep2 (leaf a) = leaf a
deep2 (node (leaf _) r) = r
deep2 (node (node l r) t) with deep2 (deep2 (node l r))
... | leaf a = leaf a
... | node l2 r2 = deep2 (node l2 r2)

-- increasing the termination count does the job!