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
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  d : DisjointAdd x s0 s'
  p : SPath Gl s0 y z
  IHp : Disjoint (snodes Gl p) (V G1) ->
        (Some (sweight p) <= lsp G2 y z)%nbar
  disj : Disjoint (VSet.add x (snodes Gl p)) (V G1)
  w : Z
  s : V.t
  w' : Z
  t : V.t
  ============================
   ((x, w, y) =
    (if EdgeSet.mem (s, w', t) (E G1)
     then ((s, w', t)..s, diff l (s, w', t)..s (s, w', t)..t, (s, w', t)..t)
     else (s, w', t)) /\ EdgeSet.In (s, w', t) (E G2) ->
    (Some w <= lsp G2 x y)%nbar)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrcase@0> $1 $2
DEBUG LTAC Evaluated term: edge_map_spec2
DEBUG LTAC Evaluated term: (iffLR
                              (Q:=exists e0 : Edge.t,
                                    (x, w, y) = relabel_map G1 l e0 /\
                                    EdgeSet.In e0 (E G2))
                              (edge_map_spec2 (relabel_map G1 l) 
                                 (E G2) (x, w, y)) _view_subject_)