Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → B → C → Type
  x : A
  y : B
  z : C
  l : list A
  l' : list B
  l'' : list C
  r : R x y z
  a : All3 R l l' l''
  r0 : R x y z
  b : All3 R l l' l''
  H : {| pr1 := r; pr2 := a |} = {| pr1 := r0; pr2 := b |}
  ============================
   (All3_cons r a = All3_cons r0 b)


DEBUG:Going to execute:
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