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


DEBUG:Going to execute:
fun id =>
             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