Add explanation of nested control flow and how to avoid it.

[BrooklinJazz]

1 -> "a" -> "1a" "b" -> "1b" 2 -> "a" -> "2a" "b" -> "2b"

vs

{1, "a"} -> "1a" {1, "b"} -> "1b" {2, "a"} -> "2a" {2, "b"} -> "2b"