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