ordinal-notations
There are 2 repositories under ordinal-notations topic.
coq-community/hydra-battles
Variations on Kirby & Paris' hydra battles and other entertaining math in Coq (collaborative, documented, includes exercises) [maintainer=@Casteran]
cj-xu/OrdinalNotations
An Agda development of ordinal notations based on Cantor normal form via simultaneous definitions