dic = {"a": 1, "b": 2}
print!(dic["c"]) # ERR

arr = [1, 2, 3]
print!(arr[5]) # ERR

# OK
for! 0..2, i =>
    print! arr[i]
# ERR
for! 0..3, i =>
    print! arr[i]
# OK
for! range(3), i =>
    print! arr[i]
# ERR
for! range(4), i =>
    print! arr[i]

concat|T: Type, M: Nat, N: Nat|(l: [T; M], r: [T; N]): [T; M + N] = l + r
l = concat [1, 2, 3], [4, 5, 6]
_ = l[6] # ERR