subst h1
case h1
v u x : VarName
phi : Formula
phi_ih : ∀ (binders : Finset VarName), fastAdmitsAux v u binders phi → admitsAux v u binders phi
binders : Finset VarName
h1 : v = x
⊢ v ∈ binders ∪ {x}
case h1
v u : VarName
phi : Formula
phi_ih : ∀ (binders : Finset VarName), fastAdmitsAux v u binders phi → admitsAux v u binders phi
binders : Finset VarName
⊢ v ∈ binders ∪ {v}