fact: Definition is_sigma (x t : term') := ∀ $0 ∈ t`[↑] <~> $0 ∈ x`[↑] ∨ $0 ≡ x`[↑].
imports: Undecidability.FOL.Util.Syntax Undecidability.FOL.Util.FullTarski Undecidability.FOL.Util.FullDeduction Undecidability.FOL.ZF List