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