𝒜 ⊨ ¬∃xF ⇔ ¬∃a ∈ A  𝒜[x→a] (F) = 1 ⇔ ∀a ∈ A  𝒜[x→a] (F) = 0
𝒜 ⊨ ∃x¬F ⇔ ∃a ∈ A  𝒜[x→a](F) = 0