Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : R x y
  rs : All2 R l l'
  r0 : R' x y r
  a0 : All2_dep R' rs
  r2 : R' x y r
  b : All2_dep R' rs
  H : {| pr1 := r0; pr2 := a0 |} = {| pr1 := r2; pr2 := b |}
  ============================
   (All2_dep_cons r0 a0 = All2_dep_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 := a0 |} =
                            {| pr1 := r2; pr2 := b |})
Level 6: type of H
 has value 
({| pr1 := r0; pr2 := a0 |} = {| pr1 := r2; pr2 := b |})