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refactor(Algebra/Star/*): Allow for star operation on non-associative…
… algebras (#6562) Typically a * operation on a mathematical structure `R` equipped with a multiplication is an involutive anti-automorphism i.e. ``` ∀ r s : R, star (r * s) = star s * star r ``` Currently mathlib defines a class `StarSemigroup` to be a semigroup satisfying this property. However, the requirement for the multiplication to be associative is unnecessarily restrictive. There are important classes of star-algebra which are not associative (e.g. JB*-algebras). This PR removes the requirement for a `StarSemigroup` to be a semigroup, merely requiring it to have a multiplication. I've changed the name from `StarSemigroup` to `StarMul` since it's no longer a semigroup. [Zulip discussion](https://leanprover.zulipchat.com/#narrow/stream/116395-maths/topic/non-associative.20*-algebras) Previously opened as a mathlib PR leanprover-community/mathlib#17949 Co-authored-by: Christopher Hoskin <mans0954@users.noreply.github.com> Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
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