@c089 Because it's what you state your xs is: a list
of numbers:

https://www.typescriptlang.org/play/index.html?ssl=7&ssc=6&pln=7&pc=15#code/MYewdgzgLgBAZjAvDAFADwgLjAVwLYBGApgE4DaAugJRIB8MA3gFAysyiSxloCMANGgBMFJDAws2JIlBwkwYngGohAbiYBfJkzgoy-GMJoB6IzBABrbbv44wAEyJwAlmCJ3qMEzFIkQJIA