Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  H : True
  ============================
   (All2_dep_nil = All2_dep_nil)


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