apply Nat.add_lt_add_left
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
⊢ as.size + i < as.size + (1 + (stop - (start + 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 < 1 + (stop - (start + 1))