AD crash

[athas]

This produces invalid code after AD:

entry scan_arr_add [n]
                   (inp: [2][n]f32)
                   (adj: [2][n]f32) : [2][n]f32 =
  let adj =
    vjp (scan (\x y -> [x[0] + y[0], x[1] + y[1]])
              (replicate 2 0))
        (transpose inp)
        (transpose adj)
  in transpose adj

The problem is that some part of the differentiation rule assumes that the operands are never arrays.