-- Lean 4 proof step from Mathlib.Data.Matrix.Invertible (goal)
Goal:
m : Type u_1
n : Type u_2
α : Type u_3
inst : Fintype n
inst_1 : DecidableEq n
inst_2 : Fintype m
inst_3 : DecidableEq m
inst_4 : Ring α
A : Matrix n n α
U : Matrix n m α
C : Matrix m m α
V : Matrix m n α
inst_5 : Invertible A
inst_6 : Invertible C
inst_7 :
  Invertible
    (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
      V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
rw :
  U *
            @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
              (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
                V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
              _ *
          V *
        @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ +
      U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U *
            @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
              (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
                V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
              _ *
          V *
        @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ =
    (U + U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U) *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
            (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
              V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
            _ *
        V *
      @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _
⊢ 1 + U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ -
      (U *
              @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
                (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
                  V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
                _ *
            V *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ +
        U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U *
              @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
                (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
                  V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
                _ *
            V *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _) =
    1 + U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ -
      (U + U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U) *
            @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
              (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
                V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
              _ *
          V *
        @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _
have rw₁ : U *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
            (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
              V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
            _ *
        V *
      @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ +
    U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
            (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
              V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
            _ *
        V *
      @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ =
  (U *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
            (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
              V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
            _ +
        U * C * V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U *
          @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one
            (@Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one C _ +
              V * @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _ * U)
            _) *
      V *
    @Invertible.invOf _ instMulOfFintypeOfAddCommMonoid one A _
Tactic: rw [sub_right_inj, Matrix.add_mul, Matrix.add_mul, Matrix.add_mul]