apply IsDeduct.assume_
F : Formula
Δ Γ : Set Formula
h1_phi : Formula
h1_1 : h1_phi ∈ Δ
⊢ IsDeduct (Δ ∪ Γ) h1_phi
case a
F : Formula
Δ Γ : Set Formula
h1_phi : Formula
h1_1 : h1_phi ∈ Δ
⊢ h1_phi ∈ Δ ∪ Γ