Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  H0 : invariants G2
  H1 : acyclic_no_loop G2
  sub : subgraph G1 G2
  ============================
   (is_full_extension G1 G2 -> full_subgraph G1 G2)


DEBUG:Going to execute:
move  =>ext; constructor => //