simp
XS YS : Finset (Finset Formula)
X Y : Finset Formula
X_in : X ∈ XS
Y_in : Y ∈ YS
⊢ X ∪ Y ∈ XS⊎YS
XS YS : Finset (Finset Formula)
X Y : Finset Formula
X_in : X ∈ XS
Y_in : Y ∈ YS
⊢ ∃ a ∈ XS, ∃ a_1 ∈ YS, X ∪ Y = a ∪ a_1