The reason the first case is correct is because Nim implicitly calls `pairs` when the for loop has exactly 2 variables.
See [https://nim-lang.org/docs/manual.html#iterators-and-the-for-statement-implict-items-pairs-invocations](https://nim-lang.org/docs/manual.html#iterators-and-the-for-statement-implict-items-pairs-invocations)
