-- Mathlib/Algebra/GroupWithZero/Basic.lean
lemma zpow_add' {m n : ℤ} (h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0) :
    a ^ (m + n) = a ^ m * a ^ n

Proof state:
case neg
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : ¬n = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
by_cases ha : a = 0

Result:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : ¬n = 0
ha : a = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

case neg
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : ¬n = 0
ha : ¬a = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Proof state:
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
by_cases hm : m = 0

Result:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : m = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

case neg
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Proof state:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : ¬n = 0
ha : a = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
subst a

Result:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
m n : ℤ
hm : ¬m = 0
hn : ¬n = 0
h : 0 ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
⊢ 0 ^ (m + n) = 0 ^ m * 0 ^ n

Proof state:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
m n : ℤ
hm : ¬m = 0
hn : ¬n = 0
h : ¬m + n = 0
⊢ 0 ^ (m + n) = 0 ^ m * 0 ^ n

Tactic:
rw [zero_zpow _ h, zero_zpow _ hm, zero_mul]

Result:
no goals

Proof state:
case neg
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
by_cases hn : n = 0

Result:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : n = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

case neg
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : ¬n = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Proof state:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : m = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
simp [hm]

Result:
no goals

Proof state:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
m n : ℤ
hm : ¬m = 0
hn : ¬n = 0
h : 0 ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
⊢ 0 ^ (m + n) = 0 ^ m * 0 ^ n

Tactic:
simp only [false_or, eq_self_iff_true, not_true, Ne, hm, hn, false_and, or_false] at h

Result:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
m n : ℤ
hm : ¬m = 0
hn : ¬n = 0
h : ¬m + n = 0
⊢ 0 ^ (m + n) = 0 ^ m * 0 ^ n

Proof state:
case pos
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : n = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
simp [hn]

Result:
no goals

Proof state:
case neg
G₀ : Type u_2
inst✝ : GroupWithZero G₀
a : G₀
m n : ℤ
h : a ≠ 0 ∨ m + n ≠ 0 ∨ m = 0 ∧ n = 0
hm : ¬m = 0
hn : ¬n = 0
ha : ¬a = 0
⊢ a ^ (m + n) = a ^ m * a ^ n

Tactic:
exact zpow_add₀ ha m n

Result:
no goals