fact: Definition is_union (x t : term') := ∀ $0 ∈' t`[↑] <~> ∃ $0 ∈' shift 2 x ∧ $1 ∈' $0.
imports: Undecidability.FOL.Util.Syntax Undecidability.FOL.Util.sig_bin Undecidability.FOL.Util.FullTarski Undecidability.FOL.Util.FullDeduction List