Goal:
    n1 : Z  y1 : V.t  x2 : V.t  n2 : Z  y2 : V.t  H : n2 < n1
  ============================
   (V.eq x2 x2 /\ n2 < n1 \/ V.eq x2 x2 /\ n2 = n1 /\ V.lt y2 y1)


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