Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type  H : True
  ============================
   (All3_nil = All3_nil)


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