llp/rlp PR

Stream: Infinity-Cosmos

[Jack McKoen]

#19135 is a PR for llp/rlp and basic closure properties (retracts, pullbacks/pushout). Let me know if you think there are other things which should be added (I'm sure there are).

This also defines retracts, but I'm not sure what file I should put them in (hopefully someone here has an idea)