Goal:
    A : Type  n : nat  hd : A  tl : list A  H : S n < #|hd :: tl|
  ============================
   (n < #|tl|)


DEBUG:Going to execute:
match goal with
           | H':?Y
             |- _ =>
                 match H with
                 | H' => fail 2
                 | _ => unify X Y; clear H' || clear H
                 end
           end