Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  Hle : leq_vertices G n x x
  Vy : VSet.In x (V G)
  Vx : ~ VSet.In x (V G)
  Hx : x = x
  XX : Z.of_nat
         (if is_left (left Hx)
          then
           if n <=? 0
           then (Z.to_nat (Z.of_nat (to_label (lsp G (s G) x)) - n) + 1)%nat
           else to_label (lsp G (s G) x)
          else to_label (lsp G (s G) x)) + n <=
       Z.of_nat
         (if is_left (left eq_refl)
          then
           if n <=? 0
           then (Z.to_nat (Z.of_nat (to_label (lsp G (s G) x)) - n) + 1)%nat
           else to_label (lsp G (s G) x)
          else to_label (lsp G (s G) x))
  ============================
   False


DEBUG:Going to execute:
<coq-core.plugins.ltac::contradiction@0> $1