apply C_18_4 (exists_ u P_u) (exists_ v P_v) Q Q' Δ h2
Q Q' P_u P_v : Formula
u v : VarName
Δ : Set Formula
h1 : IsDeduct Δ Q
h2 : IsReplOfFormulaInFormula (exists_ u P_u) (exists_ v P_v) Q Q'
h3 : Similar P_u P_v u v
⊢ IsDeduct Δ Q'
case h2
Q Q' P_u P_v : Formula
u v : VarName
Δ : Set Formula
h1 : IsDeduct Δ Q
h2 : IsReplOfFormulaInFormula (exists_ u P_u) (exists_ v P_v) Q Q'
h3 : Similar P_u P_v u v
⊢ IsProof ((exists_ u P_u).iff_ (exists_ v P_v))

case h3
Q Q' P_u P_v : Formula
u v : VarName
Δ : Set Formula
h1 : IsDeduct Δ Q
h2 : IsReplOfFormulaInFormula (exists_ u P_u) (exists_ v P_v) Q Q'
h3 : Similar P_u P_v u v
⊢ IsDeduct Δ Q