rw [size_push]
α : Type u_1
as bs : Array α
start stop : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hne : ¬start = stop
hstart' : start < stop
⊢ (as.push bs[start]).size + (stop - (start + 1)) = as.size + (stop - start)
α : Type u_1
as bs : Array α
start stop : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hne : ¬start = stop
hstart' : start < stop
⊢ as.size + 1 + (stop - (start + 1)) = as.size + (stop - start)