revert h
τ : MLIRTy
l : List TensorElem
h : hasType (nested l) τ = true
⊢ flatten (nested l) h = List.join (mapWithType l flatten h)
τ : MLIRTy
l : List TensorElem
⊢ ∀ (h : hasType (nested l) τ = true), flatten (nested l) h = List.join (mapWithType l flatten h)