Mathlib v3 is deprecated. Go to Mathlib v4

Commit 2021-04-27 07:21 40b15f25

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feat(algebra/direct_sum): state what it means for a direct sum to be internal (#7190) The goal here is primarily to tick off the item in undergrad.yml. We'll want some theorems relating this statement to independence / spanning of the submodules, but I'll leave those for someone else to follow-up with. We end up needing three variants of this - one for each of add_submonoids, add_subgroups, and submodules.

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