Goal:
  
  G1 : t
  G2 : t
  H : acyclic_no_loop G1
  H0 : invariants G1
  hext : forall l1 : labelling,
         correct_labelling G1 l1 ->
         exists l2 : labelling,
           correct_labelling G2 l2 /\
           (forall v : VSet.elt, VSet.In v (V G1) -> l1 v = l2 v)
  ============================
   (forall vx vy : VSet.elt,
    VSet.In vx (V G1) ->
    VSet.In vy (V G1) -> (lsp G2 vx vy <= lsp G1 vx vy)%nbar)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2