comp_mul_left

∀ {α : Type u_1} [inst : Semigroup α] (x y : α), ((fun x_1 => x * x_1) ∘ fun x => y * x) = fun x_1 => x * y * x_1

Composing two multiplications on the left by y then x is equal to a multiplication on the left by x * y.

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