rw [Nat.sub_succ']
case h
α : Type u_1
as bs : Array α
start stop i : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hi : i < stop - start
ha : as.size + i < (foldl push as bs start stop).size
hb : start + i < bs.size
hne : ¬start = stop
hstart' : start < stop
this : start < bs.size
⊢ i < stop - (start + 1) + 1
case h
α : Type u_1
as bs : Array α
start stop i : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hi : i < stop - start
ha : as.size + i < (foldl push as bs start stop).size
hb : start + i < bs.size
hne : ¬start = stop
hstart' : start < stop
this : start < bs.size
⊢ i < stop - start - 1 + 1