Goal:
    x1 : V.t  n1 : Z  y1 : V.t  x2 : V.t  n2 : Z  y2 : V.t  H : V.lt x1 x2
  ============================
   (V.lt x1 x2 \/
    V.eq x1 x2 /\ n1 < n2 \/ V.eq x1 x2 /\ n1 = n2 /\ V.lt y1 y2)


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]