Ltac cnf2 f :=
  match f with
   | ?A \/ (?B /\ ?C) =>
     let c1 := cnf2 (A\/B) in
     let c2 := cnf2 (A\/C) in constr:(c1/\c2)
   | (?B /\ ?C) \/ ?A =>
     let c1 := cnf2 (B\/A) in
     let c2 := cnf2 (C\/A) in constr:(c1/\c2)
   | (?A \/ ?B) \/ ?C =>
     let c1 := cnf2 (B\/C) in cnf2 (A \/ c1)
   | _ => f
  end
with cnf f :=
  match f with
   | ?A \/ ?B =>
     let c1 := cnf A in
       let c2 := cnf B in
         cnf2 (c1 \/ c2)
   | ?A /\ ?B =>
     let c1 := cnf A in
       let c2 := cnf B in
         constr:(c1 /\ c2)
   | _ => f
  end.Ltac scnf :=
  match goal with
    | |- ?f => let c := cnf f in
      assert c; [repeat split|]
  end.Definition O := PA.Definition E := PB.Definition E' := PC.

Lemma diff_f : forall A B, Col O E A -> Col O E B -> {C | Diff O E E' A B C}.