Goal:
    A : Type  n : nat  H : n < #|[]|
  ============================
   A


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