have : start < ns.size := Nat.lt_of_lt_of_le hstart hstop
ns : Array Nat
start stop : Nat
hstart : start < stop
hstop : stop ≤ ns.size
⊢ let_fun this := ⋯;
  ns.sum start stop = ns[start] + ns.sum (start + 1) stop
ns : Array Nat
start stop : Nat
hstart : start < stop
hstop : stop ≤ ns.size
this : start < ns.size
⊢ let_fun this := ⋯;
  ns.sum start stop = ns[start] + ns.sum (start + 1) stop