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chore: make RingHom.comp implicit_reducible #38315

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chore: make RingHom.comp implicit_reducible #38315

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sgouezel Apr 20, 2026
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Update Mathlib/Algebra/Ring/Hom/Defs.lean
sgouezel Apr 25, 2026
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4 changes: 3 additions & 1 deletion Mathlib/Algebra/Ring/Hom/Defs.lean
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Expand Up @@ -534,8 +534,10 @@ theorem coe_monoidHom_id : (id α : α →* α) = MonoidHom.id α :=
variable {_ : NonAssocSemiring γ}

/-- Composition of ring homomorphisms is a ring homomorphism. -/
@[implicit_reducible]
def comp (g : β →+* γ) (f : α →+* β) : α →+* γ :=
{ g.toNonUnitalRingHom.comp f.toNonUnitalRingHom with toFun := g ∘ f, map_one' := by simp }
{ g.toNonUnitalRingHom.comp f.toNonUnitalRingHom with
toFun := fun x ↦ g (f x), map_one' := by simp }
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/-- Composition of semiring homomorphisms is associative. -/
theorem comp_assoc {δ} {_ : NonAssocSemiring δ} (f : α →+* β) (g : β →+* γ) (h : γ →+* δ) :
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