let n be non empty Nat; :: thesis: for S being non empty non void n PC-correct PCLangSignature
for L being language MSAlgebra over S
for F being PC-theory of L
for A, B being Formula of L holds
( A \iff B in F iff (\not (\not A)) \iff B in F )

let S be non empty non void n PC-correct PCLangSignature ; :: thesis: for L being language MSAlgebra over S
for F being PC-theory of L
for A, B being Formula of L holds
( A \iff B in F iff (\not (\not A)) \iff B in F )

let L be language MSAlgebra over S; :: thesis: for F being PC-theory of L
for A, B being Formula of L holds
( A \iff B in F iff (\not (\not A)) \iff B in F )

let F be PC-theory of L; :: thesis: for A, B being Formula of L holds
( A \iff B in F iff (\not (\not A)) \iff B in F )

let A, B be Formula of L; :: thesis: ( A \iff B in F iff (\not (\not A)) \iff B in F )
( (\not (\not A)) \imp A in F & A \imp (\not (\not A)) in F ) by ;
then ( ((\not (\not A)) \imp A) \and (A \imp (\not (\not A))) in F & (((\not (\not A)) \imp A) \and (A \imp (\not (\not A)))) \imp ((\not (\not A)) \iff A) in F ) by ;
then A1: (\not (\not A)) \iff A in F by Def38;
then A \iff (\not (\not A)) in F by Th90;
hence ( A \iff B in F iff (\not (\not A)) \iff B in F ) by ; :: thesis: verum