axiom P4:
forall B,C in A /\ x">"y /\ x'">"y' => (x,B;y,Bc)"=>"(x,C;y,Cc)<=>(x',B;y',Bc)"=>"(x',C;y',Cc)

Define "=>*" for x">"y B"=>*" <=> (x,B;y,Bc)"=>"(x,C;y,Cc)