Goal:
  
  A : Type
  P : list A → A → Type
  l : list A
  a : All_fold P l
  b : All_fold P l
  e : NoConfusionHom_All_fold A P l a b
  ============================
   (noConfusionHom_All_fold_obligation_2 A P l a b
      (noConfusionHom_All_fold_obligation_1 A P l a b e) = e)


DEBUG:Going to execute:
match 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