Goal:
    y1 : V.t  x2 : V.t  n2 : Z  y2 : V.t  H : V.lt y2 y1
  ============================
   (n2 = n2)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end