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


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