have heq := p3 A ⟨hzmem, hind⟩
x : ↑N0
A : Set ↑N0 := {x | mul (S z, x) = x}
hzmem : z ∈ A
hind : ∀ n ∈ A, S n ∈ A
⊢ mul (S z, x) = x
x : ↑N0
A : Set ↑N0 := {x | mul (S z, x) = x}
hzmem : z ∈ A
hind : ∀ n ∈ A, S n ∈ A
heq : Subtype.val '' A = N0
⊢ mul (S z, x) = x