subst c1
v u x y : VarName
binders : Finset VarName
h1 : v ∉ binders
h2 : v = x ∨ v = y → u ∉ binders
c1 : v = y
⊢ y ∈ binders ↔ u ∈ binders
v u x : VarName
binders : Finset VarName
h1 : v ∉ binders
h2 : v = x ∨ v = v → u ∉ binders
⊢ v ∈ binders ↔ u ∈ binders