subst h2
F F' : Formula
v u : VarName
binders : Finset VarName
h1 : Rec.fastAdmitsAux v u binders F
h2 : Rec.fastReplaceFree v u F = F'
⊢ IsSub F v u F'
F : Formula
v u : VarName
binders : Finset VarName
h1 : Rec.fastAdmitsAux v u binders F
⊢ IsSub F v u (Rec.fastReplaceFree v u F)