treeMap.lean:8:59-8:69: error: failed to prove termination, possible solutions:
  - Use `have`-expressions to prove the remaining goals
  - Use `termination_by` to specify a different well-founded relation
  - Use `decreasing_by` to specify your own tactic for discharging this kind of goal
t✝ t : TreeNode
⊢ sizeOf t < sizeOf t✝