rw [Nat.sub_add_cancel this]
case e_a
α : Type u_1
as bs : Array α
start stop : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hne : ¬start = stop
hstart' : start < stop
this : 1 ≤ stop - start
⊢ stop - start - 1 + 1 = stop - start
no goals