Goal:
    A : Type  P : list A → A → Type  l : list A  a : All_fold P l
  ============================
   (NoConfusionHom_All_fold A P l a a)


DEBUG:Going to execute:
PTRDEBUGTACmatch type of id with
           | sigma ?P =>
               let idx := fresh "idx" in
               destruct id as [idx id]; repeat CoreTactics.destruct_sigma idx;
                simpl in id
           | _ => idtac
           end