Goal:
  
  y1 : V.t
  x2 : V.t
  n2 : Z
  y2 : V.t
  x3 : V.t
  n3 : Z
  y3 : V.t
  H1'' : V.lt y1 y2
  H2 : V.lt x2 x3
  T1 : V.lt x2 x2 -> V.lt x2 x3 -> V.lt x2 x3
  T3 : n2 = n2 -> n2 = n3 -> n2 = n3
  H : V.lt y2 y3 -> V.lt y1 y3
  ============================
   (V.lt x2 x3 \/ x2 = x3 /\ n2 < n3 \/ x2 = x3 /\ n2 = n3 /\ V.lt y1 y3)


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