axiom P2: forall B in A forall f,g,h,h' in F (f,B;h,Bc)">"(g,B;h,Bc) <=> (f,B;h',Bc)"=>"(g,B;h',Bc)

"=>B" == forall f,g in F, there exists h in F f"=>B"g <=> (f,A;h,Ac)"=>"(g,A;h,Ac)

B in A is null <=> forall f,g in F f"~B"g

B null <=> f=g on Ac => f~g