Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  H0 : invariants G2
  l : labelling
  HGl : correct_labelling G1 l
  embed : full_subgraph G1 G2
  acG2 : acyclic_no_loop G2
  Gl := relabel_on G1 G2 l : t
  l' := to_label ∘ lsp Gl (s Gl) : V.t -> nat
  sGl : l' (s Gl) = 0%nat
  eGl : forall e : EdgeSet.elt,
        EdgeSet.In e (E Gl) ->
        Z.of_nat (l' e..s) + e..w <= Z.of_nat (l' e..t)
  ============================
   (l' (s G2) = 0%nat)


DEBUG:Going to execute:
trivial; hnf; intros