have helper := split_wf_match a b cs
α : Type
inst✝¹ : LE α
inst✝ : DecidableRel LE.le
x y : α
zs : List α
a b : α
cs : List α
⊢ let spl := split (x :: y :: a :: b :: cs);
  List.length spl.fst < List.length (a :: b :: cs) + 2 ∧ List.length spl.snd < List.length (a :: b :: cs) + 2
α : Type
inst✝¹ : LE α
inst✝ : DecidableRel LE.le
x y : α
zs : List α
a b : α
cs : List α
helper :
  let spl := split (a :: b :: cs);
  List.length spl.fst < List.length cs + 2 ∧ List.length spl.snd < List.length cs + 2
⊢ let spl := split (x :: y :: a :: b :: cs);
  List.length spl.fst < List.length (a :: b :: cs) + 2 ∧ List.length spl.snd < List.length (a :: b :: cs) + 2