2 goals
case tree.leaf
⊢ foo tree.leaf = tree.leaf

case tree.node
a_left : tree,
a_val : ℕ,
a_right : tree,
a_ih_left : foo a_left = a_left,
a_ih_right : foo a_right = a_right
⊢ foo (tree.node a_left a_val a_right) = tree.node a_left a_val a_right
induction_naming2.lean:7:0: warning: declaration '[anonymous]' uses sorry
