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
  invGl : invariants (relabel_on G1 G2 l)
  acGl : acyclic_no_sloop (relabel_on G1 G2 l)
  ============================
   (acyclic_no_sloop Gl)


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:~ _ |- _ => solve [ case H; trivial ]
              end ])