let join l sep = match l with
  | [] -> ""
  | h :: t ->
    let s = List.fold_left (fun s e -> sprintf "%s%s%s" s sep e) "" t in
    sprintf "%s%s%s" h sep s

let constraints g =
  let nodes = G.fold_vertex (fun r l -> r :: l) g [] in
  let nodes_constraints = List.map (fun (s,t) ->
    sprintf "%s >= %s" (pprint_term s) (pprint_term t)
  ) nodes in
  let nodes_constraints_strict = List.map (fun (s,t) ->
    sprintf "%s > %s" (pprint_term s) (pprint_term t)
  ) nodes in
  let nodes_conjuct = join nodes_constraints " /\\ " in
  let nodes_disjunct_strict = join nodes_constraints_strict " \\/ " in
  let nodes_disjunct_strict = sprintf "(%s)" nodes_disjunct_strict in
  let uts = RuleSet.elements (ut_composante nodes) in
  let uts_constraints = List.map (fun (s,t) ->
    sprintf "%s >= %s" (pprint_term s) (pprint_term t)
  ) uts in
  let uts_conjuct = join uts_constraints " /\\ " in
  join [nodes_conjuct; uts_conjuct; nodes_disjunct_strict] " /\\ "