Use consistent key in split

[danog]

Reliance on the key is useful in some conditions, and the non-monotonicity of the key caused a very nasty bug in our code.

[bwoebi]

You don't actually need $k, the main thing you need is replacing yield from by foreach. (which will then just use the auto-incrementing default).

[danog]

True, but it's more explicit this way.

[kelunik]

Please cover this with a test.