Commit 2020-08-12 10:41 bfcf6400

feat(topology/opens): open_embedding_of_le (#3597) Add lemma open_embedding_of_le {U V : opens α} (i : U ≤ V) : open_embedding (set.inclusion i). Also, I was finding it quite inconvenient to have x ∈ U for U : opens X being unrelated to x ∈ (U : set X). I propose we just add the simp lemmas in this PR that unfold to the set-level membership operation. (I'd be happy to go the other way if someone has a strong opinion here; just that we pick a simp normal form for talking about membership and inclusion of opens.)