1> let a = [1,2,3]
a: Int[] = size=3 {
  [0] = 1
  [1] = 2
  [2] = 3
}
  2> a.append(4)
  3> a
$R2: Int[] = size=4 {
  [0] = 1
  [1] = 2
  [2] = 3
  [3] = 4
}