rw [foldl_step] <;> try assumption
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 : start < bs.size
⊢ (foldl push as bs start stop).data = as.data ++ bs.data.extract start stop
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 : start < bs.size
⊢ (foldl push (as.push bs[start]) bs (start + 1) stop).data = as.data ++ bs.data.extract start stop