Goal:
  
  G1 : t
  G2 : t
  l : labelling
  Gl := relabel_on G1 G2 l : t
  H : invariants G1
  preserves_edges : forall e : EdgeSet.elt,
                    EdgeSet.In e (E G1) -> EdgeSet.In e (E G2)
  HGl : correct_labelling G1 l
  H0 : invariants G2
  H1 : acyclic_no_loop G2
  embed : forall v1 v2 : VSet.elt,
          VSet.In v1 (V G1) ->
          VSet.In v2 (V G1) -> (lsp G2 v1 v2 <= lsp G1 v1 v2)%nbar
  G : t
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  d : DisjointAdd x s0 s'
  e : EdgeOf G x y
  p0 : SPath G s0 y z
  x1 : V.t
  p1 : SPath G s0 x1 z
  H2 : DisjointAdd x s0 s'
  H3 : EdgeOf G x x1
  ============================
   ({|
      pr1 := s';
      pr2 :=
        {|
          pr1 := x;
          pr2 :=
            {|
              pr1 := y;
              pr2 :=
                {|
                  pr1 := z;
                  pr2 := {| pr1 := d; pr2 := {| pr1 := e; pr2 := p0 |} |}
                |}
            |}
        |}
    |} =
    {|
      pr1 := s';
      pr2 :=
        {|
          pr1 := x;
          pr2 :=
            {|
              pr1 := x1;
              pr2 :=
                {|
                  pr1 := z;
                  pr2 := {| pr1 := H2; pr2 := {| pr1 := H3; pr2 := p1 |} |}
                |}
            |}
        |}
    |} ->
    let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    {| pr1 := {| pr1 := s0; pr2 := {| pr1 := y; pr2 := z |} |}; pr2 := p0 |} =
    {| pr1 := {| pr1 := s0; pr2 := {| pr1 := x1; pr2 := z |} |}; pr2 := p1 |})


DEBUG:Going to execute:
intro || (let H := fresh in
                     intro
                     H)
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            let H0 := CoreTactics.block in
                            {|
                              pr1 :=
                                {|
                                  pr1 := s0; pr2 := {| pr1 := y; pr2 := z |}
                                |};
                              pr2 := p0
                            |} =
                            {|
                              pr1 :=
                                {|
                                  pr1 := s0; pr2 := {| pr1 := x1; pr2 := z |}
                                |};
                              pr2 := p1
                            |})
DEBUG LTAC Evaluated term: Prop