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


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