-- Mathlib/Algebra/Colimit/Ring.lean
theorem exists_of [Nonempty ι] [IsDirected ι (· ≤ ·)] (z : DirectLimit G f) :
    ∃ i x, of G f i x = z

Proof state:
ι : Type u_1
inst✝³ : Preorder ι
G : ι → Type u_2
inst✝² : (i : ι) → CommRing (G i)
f : (i j : ι) → i ≤ j → G i → G j
inst✝¹ : Nonempty ι
inst✝ : IsDirected ι fun x1 x2 => x1 ≤ x2
z : FreeCommRing ((i : ι) × G i)
x' y' : FreeCommRing ((i : ι) × G i)
i : ι
x : G i
hx :
  (of G f i) x =
    (Ideal.Quotient.mk
        (Ideal.span
          {a |
            (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
              (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                  ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
      x'
j : ι
y : G j
hy :
  (of G f j) y =
    (Ideal.Quotient.mk
        (Ideal.span
          {a |
            (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
              (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                  ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
      y'
k : ι
hik : i ≤ k
hjk : j ≤ k
⊢ (of G f k) (f i k hik x * f j k hjk y) =
    (Ideal.Quotient.mk
        (Ideal.span
          {a |
            (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
              (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                  ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
      (x' * y')

Tactic:
rw [map_mul, of_f, of_f, hx, hy]

Result:
ι : Type u_1
inst✝³ : Preorder ι
G : ι → Type u_2
inst✝² : (i : ι) → CommRing (G i)
f : (i j : ι) → i ≤ j → G i → G j
inst✝¹ : Nonempty ι
inst✝ : IsDirected ι fun x1 x2 => x1 ≤ x2
z : FreeCommRing ((i : ι) × G i)
x' y' : FreeCommRing ((i : ι) × G i)
i : ι
x : G i
hx :
  (of G f i) x =
    (Ideal.Quotient.mk
        (Ideal.span
          {a |
            (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
              (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                  ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
      x'
j : ι
y : G j
hy :
  (of G f j) y =
    (Ideal.Quotient.mk
        (Ideal.span
          {a |
            (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
              (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                  ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
      y'
k : ι
hik : i ≤ k
hjk : j ≤ k
⊢ (Ideal.Quotient.mk
          (Ideal.span
            {a |
              (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
                (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                  (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                    ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
        x' *
      (Ideal.Quotient.mk
          (Ideal.span
            {a |
              (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
                (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                  (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                    ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
        y' =
    (Ideal.Quotient.mk
        (Ideal.span
          {a |
            (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
              (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                  ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
      (x' * y')

Proof state:
ι : Type u_1
inst✝³ : Preorder ι
G : ι → Type u_2
inst✝² : (i : ι) → CommRing (G i)
f : (i j : ι) → i ≤ j → G i → G j
inst✝¹ : Nonempty ι
inst✝ : IsDirected ι fun x1 x2 => x1 ≤ x2
z : DirectLimit G f
⊢ ∃ i x, (of G f i) x = z

Tactic:
obtain ⟨z, rfl⟩ := Ideal.Quotient.mk_surjective z

Result:
case intro
ι : Type u_1
inst✝³ : Preorder ι
G : ι → Type u_2
inst✝² : (i : ι) → CommRing (G i)
f : (i j : ι) → i ≤ j → G i → G j
inst✝¹ : Nonempty ι
inst✝ : IsDirected ι fun x1 x2 => x1 ≤ x2
z : FreeCommRing ((i : ι) × G i)
⊢ ∃ i x,
    (of G f i) x =
      (Ideal.Quotient.mk
          (Ideal.span
            {a |
              (∃ i j, ∃ (H : i ≤ j), ∃ x, FreeCommRing.of ⟨j, f i j H x⟩ - FreeCommRing.of ⟨i, x⟩ = a) ∨
                (∃ i, FreeCommRing.of ⟨i, 1⟩ - 1 = a) ∨
                  (∃ i x y, FreeCommRing.of ⟨i, x + y⟩ - (FreeCommRing.of ⟨i, x⟩ + FreeCommRing.of ⟨i, y⟩) = a) ∨
                    ∃ i x y, FreeCommRing.of ⟨i, x * y⟩ - FreeCommRing.of ⟨i, x⟩ * FreeCommRing.of ⟨i, y⟩ = a}))
        z