Goal:
    A : Type  P : list A → A → Type  H : True
  ============================
   (All_fold_nil P = All_fold_nil P)


DEBUG:Going to execute:
PTRDEBUGTACmatch type of H with
           | ?R ?x ?y =>
               let rhs := fresh in
               set (rhs := y) in *; pattern sigma rhs; clearbody rhs;
                destruct H; simpl
           end