Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  a : All2i R (S n) l r
  r2 : R n x y
  b : All2i R (S n) l r
  H : {| pr1 := r0; pr2 := a |} = {| pr1 := r2; pr2 := b |}
  ============================
   (All2i_cons r0 a = All2i_cons r2 b)


DEBUG:Going to execute:
let rhs := fresh in
           set (rhs := y) in *; pattern sigma rhs; clearbody rhs; destruct H;
            simpl
DEBUG LTAC Evaluated term: H
DEBUG LTAC Evaluated term: ({| pr1 := r0; pr2 := a |} =
                            {| pr1 := r2; pr2 := b |})
Level 6: type of H
 has value 
({| pr1 := r0; pr2 := a |} = {| pr1 := r2; pr2 := b |})