Section CGDist.
Variable cg : CGraph1.
Definition CG_edge (x y : ad) :=
match MapGet _ cg x with
| Some edges =>
match MapGet _ edges y with
| Some d => Some d
| _ => None
end
| _ => None
end.
(*s Let $\rho$ be an interpretation of adresses as elements in [D], the graph [cg] is satisfied by $\rho$ if for any edge from $x$ to $y$ indexed by $d$, we have $\rho(x) \leq \rho(y)+d$ A graph is consistent if there exists an interpretation which satisfies it. *)
Definition CGsat (rho : ad -> D) :=
forall (x y : ad) (d : D),
CG_edge x y = Some d -> Dle (rho x) (Dplus (rho y) d) = true.
Definition CGconsistent := sig CGsat.
(* [CG_path last d l] if there exists a path starting from [last] with successive vertexes $l=[x0;...;xn]$ $(xn=last)$ and [d] is the sum of the weights on the edges *)
Inductive CG_path (last : ad) : D -> list ad -> Set :=
| CG_p1 : forall x : ad, x = last -> CG_path last Dz (x :: nil)
| CG_p2 :
forall (x y : ad) (l : list ad) (d : D),
CG_path last d (y :: l) ->
forall d' : D,
CG_edge x y = Some d' -> CG_path last (Dplus d d') (x :: y :: l).
(* If $\rho$ satisfies the graph and there is a path from [last] to [x] of weight d then $\rho(x) \leq \rho(last)+d$ *)
Definition first (l : list ad) : ad :=
match l with
| nil => N0
| x :: _ => x
end.

Lemma CG_path_head :
forall (l : list ad) (last : ad) (d : D),
CG_path last d l ->
forall rho : ad -> D,
CGsat rho -> Dle (rho (first l)) (Dplus (rho last) d) = true.