// Returns characters (as strings).
IO.print("abcd"[0]) // expect: a
IO.print("abcd"[1]) // expect: b
IO.print("abcd"[2]) // expect: c
IO.print("abcd"[3]) // expect: d

// Allows indexing backwards from the end.
IO.print("abcd"[-4]) // expect: a
IO.print("abcd"[-3]) // expect: b
IO.print("abcd"[-2]) // expect: c
IO.print("abcd"[-1]) // expect: d