Goal:
    A : Type  P : list A → list A → A → A → Type  H : True
  ============================
   ({| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |} =
    {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |})


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