⊦ a ∈ ax ↔ (∃ d. a × d ∈ ax)
a ∈ (b × d0)' ↔ (∃ d. a × d ∈ (b × d0)') ⊦ a ∈ (b × d0)' ↔ a = b
Used terms: a, (b × d0)'