Lean4
/-- Makes an isomorphism of quotients by two congruence relations, given that the relations are
equal. -/
@[to_additive /-- Makes an additive isomorphism of quotients by two additive congruence relations,
given that the relations are equal. -/
]
protected def congr {c d : Con M} (h : c = d) : c.Quotient ≃* d.Quotient :=
{ Quotient.congr (Equiv.refl M) <| by apply Con.ext_iff.mp h with
map_mul' := fun x y => by rcases x with ⟨⟩; rcases y with ⟨⟩; rfl }