do(
  define(x, arr[1, map{x: 2, y: arr[3, 4]}]),
  print(element(x, 0)),         # 1
  print(<-(x, 1)),              # Map {x => 2, y => [3, 4]}
  print(x("sub", 1, "y")),      # [3, 4]
  =(x, 1, "y", 5),
  print(element(x, 1)),         # Map {x => 2, y => 5}
  =(x, 1, "x", arr[0, 1]),
  print(element(x, 1, "x"))     # [0, 1]
)