Goal:
    A : Type  E : EqDec A  x : A  y : A
  ============================
   (reflectProp (x = y) (eq_dec_to_bool x y))


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