#check_simp #[1,2,3,4,5][2]  ~> 3
#check_simp #[1,2,3,4,5][2]? ~> some 3
#check_simp #[1,2,3,4,5][7]? ~> none
#check_simp #[][0]? ~> none
#check_simp #[1,2,3,4,5][2]! ~> 3
#check_simp #[1,2,3,4,5][7]! ~> (default : Nat)
#check_simp (#[] : Array Nat)[0]! ~> (default : Nat)

attribute [local simp] Id.run in
#check_simp
  (Id.run do
    let mut s := 0
    for i in [1,2,3,4].toArray do
      s := s + i
    pure s) ~> 10

attribute [local simp] Id.run in
#check_simp
  (Id.run do
    let mut s := 0
    for h : i in [1,2,3,4].toArray do
      s := s + i
    pure s) ~> 10