simp only [c1]
z : String
t : Var
x : String
c1 : free_ z = free_ x
⊢ (free_ x).freeVarSet \ {free_ z} ⊆ t.freeVarSet
z : String
t : Var
x : String
c1 : free_ z = free_ x
⊢ (free_ x).freeVarSet \ {free_ x} ⊆ t.freeVarSet