-- Lean 4 proof step from Mathlib.GroupTheory.FreeAbelianGroup (goal)
Goal:
α : Type u
G : Type u_1
β : Type v
γ : Type w
R : Type u_2
inst : Monoid α
inst_1 : Ring R
f : α →* R
x y1 y2 : FreeAbelianGroup α
ih1 :
  (((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) : AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
        (a : FreeAbelianGroup α) → R)
      (x * y1) =
    HMul.hMul (α := R) (β := R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          x :
        R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          y1 :
        R)
ih2 :
  (((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) : AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
        (a : FreeAbelianGroup α) → R)
      (x * y2) =
    HMul.hMul (α := R) (β := R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          x :
        R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          y2 :
        R)
add :
  (((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) : AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
        (a : FreeAbelianGroup α) → R)
      (x * y1 + x * y2) =
    HMul.hMul (α := R) (β := R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          x :
        R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          (y1 + y2) :
        R)
⊢ (((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) : AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
        (a : FreeAbelianGroup α) → R)
      (x * (y1 + y2)) =
    HMul.hMul (α := R) (β := R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          x :
        R)
      ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
              AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
            (a : FreeAbelianGroup α) → R)
          (y1 + y2) :
        R)
have add₁ : HAdd.hAdd (α := R) (β := R)
    ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
            AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
          (a : FreeAbelianGroup α) → R)
        (x * y1) :
      R)
    ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
            AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
          (a : FreeAbelianGroup α) → R)
        (x * y2) :
      R) =
  HMul.hMul (α := R) (β := R)
    ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
            AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
          (a : FreeAbelianGroup α) → R)
        x :
      R)
    ((((lift : (a : α → R) → FreeAbelianGroup α →+ R) (⇑f : (a : α) → R) :
            AddMonoidHom.{u, u_2} (FreeAbelianGroup α) R) :
          (a : FreeAbelianGroup α) → R)
        (y1 + y2) :
      R)
Tactic: rw [mul_add, map_add, map_add, mul_add, ih1, ih2]