How does fin deal with negatives

Stream: general

[Henrik Böving]

While playing around with the fin type I noticed that I can e.g. do -1 : fin 3 and it will automatically turn that value into a 2. I got curious about the mechanism and digged around in the fin definition here: https://github.com/leanprover-community/lean/tree/master/library/init/data/fin but I couldn't even find something that relates to the integers. Could someone point me at the right spot?

[Yakov Pechersky]

docs#fin.comm_ring

[Yakov Pechersky]

Specifically, subtraction is docs#fin.sub and negatives are docs#fin.has_neg