Goal:
    x1 : V.t  n1 : Z  y1 : V.t  x2 : V.t  n2 : Z  y2 : V.t  H : x1 = x2
  ============================
   (CompareSpec ((x1, n1, y1) = (x2, n2, y2))
      (V.lt x1 x2 \/
       V.eq x1 x2 /\ n1 < n2 \/ V.eq x1 x2 /\ n1 = n2 /\ V.lt y1 y2)
      (V.lt x2 x1 \/
       V.eq x2 x1 /\ n2 < n1 \/ V.eq x2 x1 /\ n2 = n1 /\ V.lt y2 y1)
      match n1 ?= n2 with
      | Eq => V.compare y1 y2
      | Lt => Lt
      | Gt => Gt
      end)


DEBUG:Going to execute:
<coq-core.plugins.ltac::subst@1>