apply IsFull.node
α : Type
a : α
⊢ IsFull (BTree.node a BTree.empty BTree.empty)
case hl
α : Type
a : α
⊢ IsFull BTree.empty

case hr
α : Type
a : α
⊢ IsFull BTree.empty

case hiff
α : Type
a : α
⊢ BTree.empty = BTree.empty ↔ BTree.empty = BTree.empty