do(
  def(x, array[1, 2, 3, array[9, 8, 7]]),
  set(x, 2, 9),
  print(x),             # [ 1, 2, 9, [ 9, 8, 7 ] ]
  set(x, 3, 1, 1000),
  print(x)              # [ 1, 2, 9, [ 9, 1000, 7 ] ]
)