simp only [occursIn] at h1
case def_
v t : VarName
a✝¹ : DefName
a✝ : List VarName
h1 : ¬occursIn t (def_ a✝¹ a✝)
⊢ fastReplaceFree t v (fastReplaceFree v t (def_ a✝¹ a✝)) = def_ a✝¹ a✝
case def_
v t : VarName
a✝¹ : DefName
a✝ : List VarName
h1 : t ∉ a✝
⊢ fastReplaceFree t v (fastReplaceFree v t (def_ a✝¹ a✝)) = def_ a✝¹ a✝