Goal:
  
  x1 : V.t
  n1 : Z
  y1 : V.t
  x2 : V.t
  n2 : Z
  y2 : V.t
  x3 : V.t
  n3 : Z
  y3 : V.t
  H1 : V.lt x1 x2 \/ x1 = x2 /\ n1 < n2 \/ x1 = x2 /\ n1 = n2 /\ V.lt y1 y2
  H2 : V.lt x2 x3 \/ x2 = x3 /\ n2 < n3 \/ x2 = x3 /\ n2 = n3 /\ V.lt y2 y3
  T1 : V.lt x1 x2 -> V.lt x2 x3 -> V.lt x1 x3
  T2 : V.lt y1 y2 -> V.lt y2 y3 -> V.lt y1 y3
  T3 : n1 = n2 -> n2 = n3 -> n1 = n3
  ============================
   (V.lt x1 x3 \/ x1 = x3 /\ n1 < n3 \/ x1 = x3 /\ n1 = n3 /\ V.lt y1 y3)


DEBUG:Going to execute:
PTRDEBUGTACdestruct H1 as [H1| [[H1 H1']| [H1 [H1' H1'']]]];
            destruct H2 as [H2| [[H2 H2']| [H2 [H2' H2'']]]]