apply IsDeduct.mp_ (forall_ v (Q.iff_ P))
case h1
P Q : Formula
v : VarName
⊢ IsDeduct {forall_ v (P.iff_ Q)} ((exists_ v Q).imp_ (exists_ v P))
case h1.a
P Q : Formula
v : VarName
⊢ IsDeduct {forall_ v (P.iff_ Q)} ((forall_ v (Q.iff_ P)).imp_ ((exists_ v Q).imp_ (exists_ v P)))

case h1.a
P Q : Formula
v : VarName
⊢ IsDeduct {forall_ v (P.iff_ Q)} (forall_ v (Q.iff_ P))