open nat

inductive tree (A : Type) :=
leaf : A → tree A,
node : tree A → tree A → tree A

set_option elaborator.lift_coercions false

definition size {A : Type} (t : tree A) :=
tree.rec (λ a, 1) (λ t₁ t₂ n₁ n₂, n₁ + n₂) t

set_option elaborator.lift_coercions true

definition size {A : Type} (t : tree A) :=
tree.rec (λ a, 1) (λ t₁ t₂ n₁ n₂, n₁ + n₂) t