let H' := inferredShape_list H
l : List TensorElem
s_head : ℕ
s_tail : List ℕ
H : inferredShape (nested l) = some (s_head :: s_tail)
⊢ s_head = List.length l
l : List TensorElem
s_head : ℕ
s_tail : List ℕ
H : inferredShape (nested l) = some (s_head :: s_tail)
H' : s_head = List.length l ∧ (List.all l fun x => decide (inferredShape x = some s_tail)) = true :=
  inferredShape_list H
⊢ s_head = List.length l