obtain rfl | hb := eq_or_ne b 0
a b c : ℤ
h : a * b ^ 2 = c ^ 2
⊢ IsSquare a ∨ b = 0
case inl
a c : ℤ
h : a * 0 ^ 2 = c ^ 2
⊢ IsSquare a ∨ 0 = 0

case inr
a b c : ℤ
h : a * b ^ 2 = c ^ 2
hb : b ≠ 0
⊢ IsSquare a ∨ b = 0