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)
  s : V.t
  w : Z
  t : V.t
  ein : EdgeSet.In (s, w, t) (E G2)
  e' := from2 G1 G2 l (edges_sub G1 G2 embed) (w; ein)
     : EdgeOf (relabel_on G1 G2 l) s t
  ============================
   (Z.of_nat (l' s) +
    (if EdgeSet.mem (s, w, t) (E G1) then diff l s t else w) <=
    Z.of_nat (l' t) ->
    (if EdgeSet.mem (s, w, t) (E G1) then diff l s t else w) >= w ->
    Z.of_nat (l' s) + w <= Z.of_nat (l' t))


DEBUG:Going to execute:
lia