a b c d : ℤ
ha : a = 1 ∨ a = -1
hb : b = 1 ∨ b = -1
hc : c = 1 ∨ c = -1
hd : d = 1 ∨ d = -1
⊢ a + b = c + d ↔ a = c ∧ b = d ∨ a = d ∧ b = c
a b c d : ℤ
ha : IsUnit a
hb : IsUnit b
hc : IsUnit c
hd : IsUnit d
⊢ a + b = c + d ↔ a = c ∧ b = d ∨ a = d ∧ b = c
rw [isUnit_iff] at ha hb hc hd
no goals
a b c d : ℤ
ha : a = 1 ∨ a = -1
hb : b = 1 ∨ b = -1
hc : c = 1 ∨ c = -1
hd : d = 1 ∨ d = -1
⊢ a + b = c + d ↔ a = c ∧ b = d ∨ a = d ∧ b = c
cases ha <;> cases hb <;> cases hc <;> cases hd <;>
    subst a <;> subst b <;> subst c <;> subst d <;>
  simp