STM: support for nested boxes of nodes to model error boundaries

Dag extended to support arbitrary clusters, renamed to Property.
Vcs generalized to not impose the data hold by a Property.
Stm(VCS) names a property "a box" and imposes a topological invariant (no
overlap).  It defines 2 kind of boxes: ProofTasks (the old cluster
notion) and ErrorBound (meant to confine errors to sub-proofs).

In the meanwhile more equations added to Make(..) functors in order to
have just one Stateid.Set module around.

5 files changed, 260 insertions, 176 deletions

diff --git a/stm/dag.ml b/stm/dag.ml
index 0c7f9f34b..99e7c9264 100644
--- a/stm/dag.ml
+++ b/stm/dag.ml
@@ -8,15 +8,6 @@
 
 module type S = sig
 
-  module Cluster :
-  sig
-    type 'd t
-    val equal : 'd t -> 'd t -> bool
-    val compare : 'd t -> 'd t -> int
-    val to_string : 'd t -> string
-    val data : 'd t -> 'd
-  end
-
   type node
   module NodeSet : Set.S with type elt = node
 
@@ -31,45 +22,57 @@ module type S = sig
   val del_nodes : ('e,'i,'d) t -> NodeSet.t  -> ('e,'i,'d) t
   val all_nodes : ('e,'i,'d) t -> NodeSet.t
 
-  val iter : ('e,'i,'d) t ->
-    (node -> 'd Cluster.t option -> 'i option ->
-      (node * 'e) list -> unit) -> unit
-
-  val create_cluster : ('e,'i,'d) t -> node list -> 'd -> ('e,'i,'d) t
-  val cluster_of : ('e,'i,'d) t -> node -> 'd Cluster.t option
-  val del_cluster : ('e,'i,'d) t -> 'd Cluster.t -> ('e,'i,'d) t
-
   val get_info : ('e,'i,'d) t -> node -> 'i option
   val set_info : ('e,'i,'d) t -> node -> 'i -> ('e,'i,'d) t
   val clear_info : ('e,'i,'d) t -> node -> ('e,'i,'d) t
 
+  module Property :
+  sig
+    type 'd t
+    val equal : 'd t -> 'd t -> bool
+    val compare : 'd t -> 'd t -> int
+    val to_string : 'd t -> string
+    val data : 'd t -> 'd
+    val having_it : 'd t -> NodeSet.t
+  end
+
+  val create_property : ('e,'i,'d) t -> node list -> 'd -> ('e,'i,'d) t
+  val property_of : ('e,'i,'d) t -> node -> 'd Property.t list
+  val del_property : ('e,'i,'d) t -> 'd Property.t -> ('e,'i,'d) t
+
+  val iter : ('e,'i,'d) t ->
+    (node -> 'd Property.t list -> 'i option ->
+      (node * 'e) list -> unit) -> unit
+
 end
 
 module Make(OT : Map.OrderedType) = struct
 
-module Cluster =
+module NodeSet = Set.Make(OT)
+
+module Property =
 struct
-  type 'd t = 'd * int
-  let equal (_,i1) (_,i2) = Int.equal i1 i2
-  let compare (_,i1) (_,i2) = Int.compare i1 i2
-  let to_string (_,i) = string_of_int i
-  let data (d,_) = d
+  type 'd t = { data : 'd; uid : int; having_it : NodeSet.t }
+  let equal { uid = i1 } { uid = i2 } = Int.equal i1 i2
+  let compare { uid = i1 } { uid = i2 } = Int.compare i1 i2
+  let to_string { uid = i } = string_of_int i
+  let data { data = d } = d
+  let having_it { having_it } = having_it
 end
 
 type node = OT.t
 
 module NodeMap = CMap.Make(OT)
-module NodeSet = Set.Make(OT)
 
 type ('edge,'info,'data) t = {
   graph : (node * 'edge) list NodeMap.t;
-  clusters : 'data Cluster.t NodeMap.t;
+  properties : 'data Property.t list NodeMap.t;
   infos : 'info NodeMap.t;
 }
 
 let empty = {
   graph = NodeMap.empty;
-  clusters = NodeMap.empty;
+  properties = NodeMap.empty;
   infos = NodeMap.empty;
 }
 
@@ -94,7 +97,7 @@ let del_edge dag id tgt = { dag with
 
 let del_nodes dag s = {
   infos = NodeMap.filter (fun n _ -> not(NodeSet.mem n s)) dag.infos;
-  clusters = NodeMap.filter (fun n _ -> not(NodeSet.mem n s)) dag.clusters;
+  properties = NodeMap.filter (fun n _ -> not(NodeSet.mem n s)) dag.properties;
   graph = NodeMap.filter (fun n l ->
       let drop = NodeSet.mem n s in
       if not drop then
@@ -102,20 +105,31 @@ let del_nodes dag s = {
       not drop)
     dag.graph }
 
+let map_add_list k v m =
+  try
+    let l = NodeMap.find k m in
+    NodeMap.add k (v::l) m
+  with Not_found -> NodeMap.add k [v] m
+
 let clid = ref 1
-let create_cluster dag l data =
+let create_property dag l data =
   incr clid;
-  { dag with clusters =
-     List.fold_right (fun x clusters ->
-       NodeMap.add x (data, !clid) clusters) l dag.clusters }
-
-let cluster_of dag id =
-  try Some (NodeMap.find id dag.clusters)
-  with Not_found -> None
-
-let del_cluster dag c =
-  { dag with clusters =
-     NodeMap.filter (fun _ c' -> not (Cluster.equal c' c)) dag.clusters }
+  let having_it = List.fold_right NodeSet.add l NodeSet.empty in
+  let property = { Property.data; uid = !clid; having_it } in
+  { dag with properties =
+     List.fold_right (fun x ps -> map_add_list x property ps) l dag.properties }
+
+let property_of dag id =
+  try NodeMap.find id dag.properties
+  with Not_found -> []
+
+let del_property dag c =
+  { dag with properties =
+     NodeMap.filter (fun _ cl -> cl <> [])
+       (NodeMap.map (fun cl ->
+          List.filter (fun c' ->
+            not (Property.equal c' c)) cl)
+          dag.properties) }
 
 let get_info dag id =
   try Some (NodeMap.find id dag.infos)
@@ -126,7 +140,7 @@ let set_info dag id info = { dag with infos = NodeMap.add id info dag.infos }
 let clear_info dag id = { dag with infos = NodeMap.remove id dag.infos }
 
 let iter dag f =
-  NodeMap.iter (fun k v -> f k (cluster_of dag k) (get_info dag k) v) dag.graph
+  NodeMap.iter (fun k v -> f k (property_of dag k) (get_info dag k) v) dag.graph
 
 let all_nodes dag = NodeMap.domain dag.graph
 
diff --git a/stm/dag.mli b/stm/dag.mli
index 6b4442df0..e783cd8ee 100644
--- a/stm/dag.mli
+++ b/stm/dag.mli
@@ -7,19 +7,7 @@
 (************************************************************************)
 
 module type S = sig
-
-  (* A cluster is just a set of nodes.  This set holds some data.
-     Stm uses this to group nodes contribution to the same proofs and
-     that can be evaluated asynchronously *)
-  module Cluster :
-  sig
-    type 'd t
-    val equal : 'd t -> 'd t -> bool
-    val compare : 'd t -> 'd t -> int
-    val to_string : 'd t -> string
-    val data : 'd t -> 'd
-  end
-
+  
   type node
   module NodeSet : Set.S with type elt = node
 
@@ -34,19 +22,35 @@ module type S = sig
   val del_nodes : ('e,'i,'d) t -> NodeSet.t  -> ('e,'i,'d) t
   val all_nodes : ('e,'i,'d) t -> NodeSet.t
 
-  val iter : ('e,'i,'d) t -> 
-    (node -> 'd Cluster.t option -> 'i option ->
-      (node * 'e) list -> unit) -> unit
-
-  val create_cluster : ('e,'i,'d) t -> node list -> 'd -> ('e,'i,'d) t
-  val cluster_of : ('e,'i,'d) t -> node -> 'd Cluster.t option
-  val del_cluster : ('e,'i,'d) t -> 'd Cluster.t -> ('e,'i,'d) t
-
   val get_info : ('e,'i,'d) t -> node -> 'i option
   val set_info : ('e,'i,'d) t -> node -> 'i -> ('e,'i,'d) t
   val clear_info : ('e,'i,'d) t -> node -> ('e,'i,'d) t
 
-end
+  (* A property applies to a set of nodes and holds some data.
+     Stm uses this feature to group nodes contributing to the same proofs and
+     to structure proofs in boxes limiting the scope of errors *)
+  module Property :
+  sig
+    type 'd t
+    val equal : 'd t -> 'd t -> bool
+    val compare : 'd t -> 'd t -> int
+    val to_string : 'd t -> string
+    val data : 'd t -> 'd
+    val having_it : 'd t -> NodeSet.t
+  end
+
+  val create_property : ('e,'i,'d) t -> node list -> 'd -> ('e,'i,'d) t
+  val property_of : ('e,'i,'d) t -> node -> 'd Property.t list
+  val del_property : ('e,'i,'d) t -> 'd Property.t -> ('e,'i,'d) t
+
+  val iter : ('e,'i,'d) t -> 
+    (node -> 'd Property.t list -> 'i option ->
+      (node * 'e) list -> unit) -> unit
+
+  end
 
-module Make(OT : Map.OrderedType) : S with type node = OT.t
+module Make(OT : Map.OrderedType) : S
+with type node = OT.t
+and type NodeSet.t = Set.Make(OT).t
+and type NodeSet.elt = OT.t
 
diff --git a/stm/stm.ml b/stm/stm.ml
index 1dd089a26..1c8603f0f 100644
--- a/stm/stm.ml
+++ b/stm/stm.ml
@@ -140,7 +140,7 @@ let update_global_env () =
   if Proof_global.there_are_pending_proofs () then
     Proof_global.update_global_env ()
 
-module Vcs_ = Vcs.Make(Stateid)
+module Vcs_ = Vcs.Make(Stateid.Self)
 type future_proof = Proof_global.closed_proof_output Future.computation
 type proof_mode = string
 type depth = int
@@ -206,15 +206,28 @@ type 'vcs state_info = { (* TODO: Make this record private to VCS *)
 let default_info () =
   { n_reached = 0; n_goals = 0; state = Empty; vcs_backup = None,None }
 
+(* Clusters of nodes implemented as Dag properties.  While Dag and Vcs impose
+ * no constraint on properties, here we impose boxes to be non overlapping. *)
+type box =
+  | ProofTask of pt
+  | ErrorBound of eb
+and pt = { (* TODO: inline records in OCaml 4.03 *)
+  start : Stateid.t;
+  qed   : Stateid.t;
+}
+and eb = {
+  stuff : unit;
+}
+
 (* Functions that work on a Vcs with a specific branch type *)
 module Vcs_aux : sig
 
-  val proof_nesting : (branch_type, 't,'i) Vcs_.t -> int
+  val proof_nesting : (branch_type, 't,'i,'c) Vcs_.t -> int
   val find_proof_at_depth :
-    (branch_type, 't, 'i) Vcs_.t -> int ->
+          (branch_type, 't, 'i,'c) Vcs_.t -> int ->
       Vcs_.Branch.t * branch_type Vcs_.branch_info
   exception Expired
-  val visit : (branch_type, transaction,'i) Vcs_.t -> Vcs_.Dag.node -> visit
+  val visit : (branch_type, transaction,'i,'c) Vcs_.t -> Vcs_.Dag.node -> visit
 
 end = struct (* {{{ *)
 
@@ -278,7 +291,7 @@ module VCS : sig
     pos  : id;
   }
 
-  type vcs = (branch_type, transaction, vcs state_info) Vcs_.t
+  type vcs = (branch_type, transaction, vcs state_info, box) Vcs_.t
 
   val init : id -> unit
 
@@ -296,7 +309,7 @@ module VCS : sig
   val edit_branch : Branch.t
   val branch : ?root:id -> ?pos:id -> Branch.t -> branch_type -> unit
   val reset_branch : Branch.t -> id -> unit
-  val reachable : id -> Vcs_.NodeSet.t
+  val reachable : id -> Stateid.Set.t
   val cur_tip : unit -> id
 
   val get_info : id -> vcs state_info
@@ -309,9 +322,11 @@ module VCS : sig
   val slice : start:id -> stop:id -> vcs
   val nodes_in_slice : start:id -> stop:id -> Stateid.t list
   
-  val create_cluster : id list -> qed:id -> start:id -> unit
-  val cluster_of : id -> (id * id) option
-  val delete_cluster_of : id -> unit
+  val create_proof_task_box : id list -> qed:id -> start:id -> unit
+  val create_error_bound_box : id list -> switch:id -> unit
+  val box_of : id -> box list
+  val delete_proof_task_box_of : id -> unit
+  val proof_task_box_of : id -> pt option
 
   val proof_nesting : unit -> int
   val checkout_shallowest_proof_branch : unit -> unit
@@ -331,7 +346,6 @@ end = struct (* {{{ *)
   include Vcs_
   exception Expired = Vcs_aux.Expired
 
-  module StateidSet = Set.Make(Stateid)
   open Printf
 
   let print_dag vcs () =
@@ -373,8 +387,6 @@ end = struct (* {{{ *)
     let edge tr =
       sprintf "<<FONT POINT-SIZE=\"12\" FACE=\"sans\">%s</FONT>>"
         (quote (string_of_transaction tr)) in
-    let ids = ref StateidSet.empty in
-    let clus = Hashtbl.create 13 in
     let node_info id =
       match get_info vcs id with
       | None -> ""
@@ -387,39 +399,66 @@ end = struct (* {{{ *)
     let nodefmt oc id =
       fprintf oc "%s [label=%s,style=filled,fillcolor=%s];\n"
         (node id) (node_info id) (color id) in
-    let add_to_clus_or_ids from cf =
-      match cf with
-      | None -> ids := StateidSet.add from !ids; false
-      | Some c -> Hashtbl.replace clus c
-         (try let n = Hashtbl.find clus c in from::n
-         with Not_found -> [from]); true in
+
+    let ids = ref Stateid.Set.empty in
+    let boxes = ref [] in
+    (* Fill in *)
+    Dag.iter graph (fun from _ _ l ->
+      ids := Stateid.Set.add from !ids;
+      List.iter (fun box -> boxes := box :: !boxes)
+        (Dag.property_of graph from);
+      List.iter (fun (dest, _) ->
+        ids := Stateid.Set.add dest !ids;
+        List.iter (fun box -> boxes := box :: !boxes)
+          (Dag.property_of graph dest))
+      l);
+    boxes := CList.sort_uniquize Dag.Property.compare !boxes;
+
     let oc = open_out fname_dot in
     output_string oc "digraph states {\n";
     Dag.iter graph (fun from cf _ l ->
-      let c1 = add_to_clus_or_ids from cf in
       List.iter (fun (dest, trans) ->
-       let c2 = add_to_clus_or_ids dest (Dag.cluster_of graph dest) in
-       fprintf oc "%s -> %s [xlabel=%s,labelfloat=%b];\n"
-           (node from) (node dest) (edge trans) (c1 && c2)) l
+       fprintf oc "%s -> %s [xlabel=%s,labelfloat=true];\n"
+           (node from) (node dest) (edge trans)) l
     );
-    StateidSet.iter (nodefmt oc) !ids;
-    Hashtbl.iter (fun c nodes ->
-       fprintf oc "subgraph cluster_%s {\n" (Dag.Cluster.to_string c);
-       List.iter (nodefmt oc) nodes;
-       fprintf oc "color=blue; }\n"
-    ) clus;
+
+    let contains b1 b2 =
+      Stateid.Set.subset
+        (Dag.Property.having_it b2) (Dag.Property.having_it b1) in
+    let same_box = Dag.Property.equal in
+    let outerboxes boxes =
+       List.filter (fun b ->
+         not (List.exists (fun b1 ->
+           not (same_box b1 b) && contains b1 b) boxes)           
+         ) boxes in
+    let rec rec_print b =
+      boxes := CList.remove same_box b !boxes;
+      let sub_boxes = List.filter (contains b) (outerboxes !boxes) in
+      fprintf oc "subgraph cluster_%s {\n" (Dag.Property.to_string b);
+      List.iter rec_print sub_boxes;
+      Stateid.Set.iter (fun id ->
+        if Stateid.Set.mem id !ids then begin
+          ids := Stateid.Set.remove id !ids;
+          nodefmt oc id
+        end)
+        (Dag.Property.having_it b);
+      fprintf oc "color=blue; }\n" in
+    List.iter rec_print (outerboxes !boxes);
+    Stateid.Set.iter (nodefmt oc) !ids;
+
     List.iteri (fun i (b,id) ->
       let shape = if Branch.equal head b then "box3d" else "box" in
       fprintf oc "b%d -> %s;\n" i (node id);
       fprintf oc "b%d [shape=%s,label=\"%s\"];\n" i shape
         (Branch.to_string b);
     ) heads;
+
     output_string oc "}\n";
     close_out oc;
     ignore(Sys.command
       ("dot -Tpdf -Gcharset=latin1 " ^ fname_dot ^ " -o" ^ fname_ps))
 
-  type vcs = (branch_type, transaction, vcs state_info) t
+  type vcs = (branch_type, transaction, vcs state_info, box) t
   let vcs : vcs ref = ref (empty Stateid.dummy)
 
   let init id =
@@ -535,15 +574,39 @@ end = struct (* {{{ *)
   let nodes_in_slice ~start ~stop =
     List.rev (List.map fst (nodes_in_slice ~start ~stop))
 
-  let create_cluster l ~qed ~start = vcs := create_cluster !vcs l (qed,start)
-  let cluster_of id = Option.map Dag.Cluster.data (cluster_of !vcs id)
-  let delete_cluster_of id =
-    Option.iter (fun x -> vcs := delete_cluster !vcs x) (Vcs_.cluster_of !vcs id)
+  let topo_invariant l =
+    let all = List.fold_right Stateid.Set.add l Stateid.Set.empty in
+    List.for_all
+      (fun x ->
+         let props = property_of !vcs x in
+         let sets = List.map Dag.Property.having_it props in
+         List.for_all (fun s -> Stateid.Set.(subset s all || subset all s)) sets)
+      l
+
+  let create_proof_task_box l ~qed ~start =
+    if not (topo_invariant l) then anomaly (str "overlapping boxes");
+    vcs := create_property !vcs l (ProofTask { qed; start })
+  let create_error_bound_box l ~switch =
+    vcs := create_property !vcs l
+      (ErrorBound { stuff = () })
+  let box_of id = List.map Dag.Property.data (property_of !vcs id)
+  let delete_proof_task_box_of id =
+    List.iter (fun x -> vcs := delete_property !vcs x)
+      (List.filter (fun x ->
+          match Dag.Property.data x with ProofTask _ -> true | _ -> false)
+        (Vcs_.property_of !vcs id))
+  let proof_task_box_of id =
+    match
+      CList.map_filter (function ProofTask x -> Some x | _ -> None) (box_of id)
+    with
+    | [] -> None
+    | [x] -> Some x
+    | _ -> anomaly (str "node with more than 1 proof task box")
 
   let gc () =
     let old_vcs = !vcs in
     let new_vcs, erased_nodes = gc old_vcs in
-    Vcs_.NodeSet.iter (fun id ->
+    Stateid.Set.iter (fun id ->
         match (Vcs_aux.visit old_vcs id).step with
         | `Qed ({ fproof = Some (_, cancel_switch) }, _)
         | `Cmd { cqueue = `TacQueue cancel_switch }
@@ -1878,7 +1941,7 @@ let known_state ?(redefine_qed=false) ~cache id =
                 let drop_pt = keep == VtKeepAsAxiom in
                 let stop, exn_info, loc = eop, (id, eop), x.loc in
                 log_processing_async id name;
-                VCS.create_cluster nodes ~qed:id ~start;
+                VCS.create_proof_task_box nodes ~qed:id ~start;
                 begin match brinfo, qed.fproof with
                 | { VCS.kind = `Edit _ }, None -> assert false
                 | { VCS.kind = `Edit (_,_,_, okeep, _) }, Some (ofp, cancel) ->
@@ -2403,7 +2466,7 @@ let edit_at id =
     | _ -> assert false
   in
   let is_ancestor_of_cur_branch id =
-    Vcs_.NodeSet.mem id
+    Stateid.Set.mem id
       (VCS.reachable (VCS.get_branch_pos (VCS.current_branch ()))) in
   let has_failed qed_id =
     match VCS.visit qed_id with
@@ -2418,13 +2481,13 @@ let edit_at id =
       | { next } -> master_for_br root next in
   let reopen_branch start at_id mode qed_id tip old_branch =
     let master_id, cancel_switch, keep =
-      (* Hum, this should be the real start_id in the clusted and not next *)
+      (* Hum, this should be the real start_id in the cluster and not next *)
       match VCS.visit qed_id with
       | { step = `Qed ({ fproof = Some (_,cs); keep },_) } -> start, cs, keep
-      | _ -> anomaly (str "Cluster not ending with Qed") in
+      | _ -> anomaly (str "ProofTask not ending with Qed") in
     VCS.branch ~root:master_id ~pos:id
       VCS.edit_branch (`Edit (mode, qed_id, master_id, keep, old_branch));
-    VCS.delete_cluster_of id;
+    VCS.delete_proof_task_box_of id;
     cancel_switch := true;
     Reach.known_state ~cache:(interactive ()) id;
     VCS.checkout_shallowest_proof_branch ();
@@ -2438,14 +2501,14 @@ let edit_at id =
     let { mine = brname, brinfo; others } = Backtrack.branches_of id in
     List.iter (fun (name,{ VCS.kind = k; root; pos }) ->
       if not(VCS.Branch.equal name VCS.Branch.master) &&
-         Vcs_.NodeSet.mem root ancestors then
+         Stateid.Set.mem root ancestors then
         VCS.branch ~root ~pos name k)
       others;
     VCS.reset_branch VCS.Branch.master (master_for_br brinfo.VCS.root id);
     VCS.branch ~root:brinfo.VCS.root ~pos:brinfo.VCS.pos
       (Option.default brname bn)
       (no_edit brinfo.VCS.kind);
-    VCS.delete_cluster_of id;
+    VCS.delete_proof_task_box_of id;
     VCS.gc ();
     VCS.print ();
     if not !Flags.async_proofs_full then
@@ -2460,14 +2523,14 @@ let edit_at id =
         | Some{ mine = bn, { VCS.kind = `Proof(m,_) }} -> Some(m,bn)
         | Some{ mine = _, { VCS.kind = `Edit(m,_,_,_,bn) }} -> Some (m,bn)
         | _ -> None in
-      match focused, VCS.cluster_of id, branch_info with
+      match focused, VCS.proof_task_box_of id, branch_info with
       | _, Some _, None -> assert false
-      | false, Some (qed_id,start), Some(mode,bn) ->
+      | false, Some { qed = qed_id ; start }, Some(mode,bn) ->
           let tip = VCS.cur_tip () in
           if has_failed qed_id && is_pure qed_id && not !Flags.async_proofs_never_reopen_branch
           then reopen_branch start id mode qed_id tip bn
           else backto id (Some bn)
-      | true, Some (qed_id,_), Some(mode,bn) ->
+      | true, Some { qed = qed_id }, Some(mode,bn) ->
           if on_cur_branch id then begin
             assert false
           end else if is_ancestor_of_cur_branch id then begin
diff --git a/stm/vcs.ml b/stm/vcs.ml
index 38c029901..c483ea4a9 100644
--- a/stm/vcs.ml
+++ b/stm/vcs.ml
@@ -22,51 +22,49 @@ module type S = sig
   end
 
   type id
-
-  (* Branches have [branch_info] attached to them. *)
+  
   type ('kind) branch_info = {
     kind : [> `Master] as 'kind;
     root : id;
     pos  : id;
   }
-
-  type ('kind,'diff,'info) t constraint 'kind = [> `Master ]
-
-  val empty : id -> ('kind,'diff,'info) t
-
-  val current_branch : ('k,'e,'i) t -> Branch.t
-  val branches : ('k,'e,'i) t -> Branch.t list
-
-  val get_branch : ('k,'e,'i) t -> Branch.t -> 'k branch_info
-  val reset_branch : ('k,'e,'i) t -> Branch.t -> id -> ('k,'e,'i) t
+  
+  type ('kind,'diff,'info,'property_data) t constraint 'kind = [> `Master ]
+  
+  val empty : id -> ('kind,'diff,'info,'property_data) t
+  
+  val current_branch : ('k,'e,'i,'c) t -> Branch.t
+  val branches : ('k,'e,'i,'c) t -> Branch.t list
+  
+  val get_branch : ('k,'e,'i,'c) t -> Branch.t -> 'k branch_info
+  val reset_branch : ('k,'e,'i,'c) t -> Branch.t -> id -> ('k,'e,'i,'c) t
   val branch :
-    ('kind,'e,'i) t -> ?root:id -> ?pos:id ->
-        Branch.t -> 'kind -> ('kind,'e,'i) t
-  val delete_branch : ('k,'e,'i) t -> Branch.t -> ('k,'e,'i) t
+    ('kind,'e,'i,'c) t -> ?root:id -> ?pos:id ->
+        Branch.t -> 'kind -> ('kind,'e,'i,'c) t
+  val delete_branch : ('k,'e,'i,'c) t -> Branch.t -> ('k,'e,'i,'c) t
   val merge :
-    ('k,'diff,'i) t -> id -> ours:'diff -> theirs:'diff -> ?into:Branch.t ->
-            Branch.t -> ('k,'diff,'i) t
-  val commit : ('k,'diff,'i) t -> id -> 'diff -> ('k,'diff,'i) t
+    ('k,'diff,'i,'c) t -> id -> ours:'diff -> theirs:'diff -> ?into:Branch.t ->
+            Branch.t -> ('k,'diff,'i,'c) t
+  val commit : ('k,'diff,'i,'c) t -> id -> 'diff -> ('k,'diff,'i,'c) t
   val rewrite_merge :
-    ('k,'diff,'i) t -> id -> ours:'diff -> theirs:'diff -> at:id ->
-            Branch.t -> ('k,'diff,'i) t
-  val checkout : ('k,'e,'i) t -> Branch.t -> ('k,'e,'i) t
-
-  val set_info : ('k,'e,'info) t -> id -> 'info -> ('k,'e,'info) t
-  val get_info : ('k,'e,'info) t -> id -> 'info option
+    ('k,'diff,'i,'c) t -> id -> ours:'diff -> theirs:'diff -> at:id ->
+            Branch.t -> ('k,'diff,'i,'c) t
+  val checkout : ('k,'e,'i,'c) t -> Branch.t -> ('k,'e,'i,'c) t
   
-  module NodeSet : Set.S with type elt = id
-
-  val gc : ('k,'e,'info) t -> ('k,'e,'info) t * NodeSet.t
-
-  val reachable : ('k,'e,'info) t -> id -> NodeSet.t
+  val set_info : ('k,'e,'info,'c) t -> id -> 'info -> ('k,'e,'info,'c) t
+  val get_info : ('k,'e,'info,'c) t -> id -> 'info option
 
+  (* Read only dag *)
   module Dag : Dag.S with type node = id
-  val dag : ('kind,'diff,'info) t -> ('diff,'info,id*id) Dag.t
+  val dag : ('kind,'diff,'info,'cdata) t -> ('diff,'info,'cdata) Dag.t
+
+  val create_property : ('k,'e,'i,'c) t -> id list -> 'c -> ('k,'e,'i,'c) t
+  val property_of : ('k,'e,'i,'c) t -> id -> 'c Dag.Property.t list
+  val delete_property : ('k,'e,'i,'c) t -> 'c Dag.Property.t -> ('k,'e,'i,'c) t 
 
-  val create_cluster : ('k,'e,'i) t -> id list -> (id * id) -> ('k,'e,'i) t
-  val cluster_of : ('k,'e,'i) t -> id -> (id * id) Dag.Cluster.t option
-  val delete_cluster : ('k,'e,'i) t -> (id * id) Dag.Cluster.t -> ('k,'e,'i) t 
+  (* Removes all unreachable nodes and returns them *)
+  val gc : ('k,'e,'info,'c) t -> ('k,'e,'info,'c) t * Dag.NodeSet.t
+  val reachable : ('k,'e,'info,'c) t -> id -> Dag.NodeSet.t
 
 end
 
@@ -78,7 +76,6 @@ type id = OT.t
 
 module NodeSet = Dag.NodeSet
 
-
 module Branch =
 struct
   type t = string
@@ -99,10 +96,10 @@ type 'kind branch_info = {
   pos  : id;
 }
 
-type ('kind,'edge,'info) t = {
+type ('kind,'edge,'info,'property_data) t = {
   cur_branch   : Branch.t;
   heads        : 'kind branch_info BranchMap.t;
-  dag          : ('edge,'info,id*id) Dag.t;
+  dag          : ('edge,'info,'property_data) Dag.t;
 }
 
 let empty root = {
@@ -167,9 +164,9 @@ let checkout vcs name = { vcs with cur_branch = name }
 let set_info vcs id info = { vcs with dag = Dag.set_info vcs.dag id info }
 let get_info vcs id = Dag.get_info vcs.dag id
 
-let create_cluster vcs l i = { vcs with dag = Dag.create_cluster vcs.dag l i }
-let cluster_of vcs i = Dag.cluster_of vcs.dag i
-let delete_cluster vcs c = { vcs with dag = Dag.del_cluster vcs.dag c }
+let create_property vcs l i = { vcs with dag = Dag.create_property vcs.dag l i }
+let property_of vcs i = Dag.property_of vcs.dag i
+let delete_property vcs c = { vcs with dag = Dag.del_property vcs.dag c }
 
 let branches vcs = BranchMap.fold (fun x _ accu -> x :: accu) vcs.heads []
 let dag vcs = vcs.dag
diff --git a/stm/vcs.mli b/stm/vcs.mli
index 8f22fee84..46b40f8a4 100644
--- a/stm/vcs.mli
+++ b/stm/vcs.mli
@@ -19,10 +19,11 @@
    As a consequence, "checkout" just updates the current branch.
 
    The type [id] is the type of commits (a node in the dag)
-   The type [Vcs.t] has 3 parameters:
+   The type [Vcs.t] has 4 parameters:
     ['info] data attached to a node (like a system state)
     ['diff] data attached to an edge (the commit content, a "patch")
     ['kind] extra data attached to a branch (like being the master branch)
+    ['cdata] extra data hold by dag properties
 *)
 
 module type S = sig
@@ -45,46 +46,51 @@ module type S = sig
     pos  : id;
   }
   
-  type ('kind,'diff,'info) t constraint 'kind = [> `Master ]
+  type ('kind,'diff,'info,'property_data) t constraint 'kind = [> `Master ]
   
-  val empty : id -> ('kind,'diff,'info) t
+  val empty : id -> ('kind,'diff,'info,'property_data) t
   
-  val current_branch : ('k,'e,'i) t -> Branch.t
-  val branches : ('k,'e,'i) t -> Branch.t list
+  val current_branch : ('k,'e,'i,'c) t -> Branch.t
+  val branches : ('k,'e,'i,'c) t -> Branch.t list
   
-  val get_branch : ('k,'e,'i) t -> Branch.t -> 'k branch_info
-  val reset_branch : ('k,'e,'i) t -> Branch.t -> id -> ('k,'e,'i) t
+  val get_branch : ('k,'e,'i,'c) t -> Branch.t -> 'k branch_info
+  val reset_branch : ('k,'e,'i,'c) t -> Branch.t -> id -> ('k,'e,'i,'c) t
   val branch :
-    ('kind,'e,'i) t -> ?root:id -> ?pos:id ->
-        Branch.t -> 'kind -> ('kind,'e,'i) t
-  val delete_branch : ('k,'e,'i) t -> Branch.t -> ('k,'e,'i) t
+    ('kind,'e,'i,'c) t -> ?root:id -> ?pos:id ->
+        Branch.t -> 'kind -> ('kind,'e,'i,'c) t
+  val delete_branch : ('k,'e,'i,'c) t -> Branch.t -> ('k,'e,'i,'c) t
   val merge :
-    ('k,'diff,'i) t -> id -> ours:'diff -> theirs:'diff -> ?into:Branch.t ->
-            Branch.t -> ('k,'diff,'i) t
-  val commit : ('k,'diff,'i) t -> id -> 'diff -> ('k,'diff,'i) t
+    ('k,'diff,'i,'c) t -> id -> ours:'diff -> theirs:'diff -> ?into:Branch.t ->
+            Branch.t -> ('k,'diff,'i,'c) t
+  val commit : ('k,'diff,'i,'c) t -> id -> 'diff -> ('k,'diff,'i,'c) t
   val rewrite_merge :
-    ('k,'diff,'i) t -> id -> ours:'diff -> theirs:'diff -> at:id ->
-            Branch.t -> ('k,'diff,'i) t
-  val checkout : ('k,'e,'i) t -> Branch.t -> ('k,'e,'i) t
+    ('k,'diff,'i,'c) t -> id -> ours:'diff -> theirs:'diff -> at:id ->
+            Branch.t -> ('k,'diff,'i,'c) t
+  val checkout : ('k,'e,'i,'c) t -> Branch.t -> ('k,'e,'i,'c) t
   
-  val set_info : ('k,'e,'info) t -> id -> 'info -> ('k,'e,'info) t
-  val get_info : ('k,'e,'info) t -> id -> 'info option
+  val set_info : ('k,'e,'info,'c) t -> id -> 'info -> ('k,'e,'info,'c) t
+  val get_info : ('k,'e,'info,'c) t -> id -> 'info option
 
-  module NodeSet : Set.S with type elt = id
+  (* Read only dag *)
+  module Dag : Dag.S with type node = id
+  val dag : ('kind,'diff,'info,'cdata) t -> ('diff,'info,'cdata) Dag.t
   
-  (* Removes all unreachable nodes and returns them *)
-  val gc : ('k,'e,'info) t -> ('k,'e,'info) t * NodeSet.t
+  (* Properties are not a concept typical of a VCS, but a useful metadata
+   * of a DAG (or graph). *)
+  val create_property : ('k,'e,'i,'c) t -> id list -> 'c -> ('k,'e,'i,'c) t
+  val property_of : ('k,'e,'i,'c) t -> id -> 'c Dag.Property.t list
+  val delete_property : ('k,'e,'i,'c) t -> 'c Dag.Property.t -> ('k,'e,'i,'c) t 
 
-  val reachable : ('k,'e,'info) t -> id -> NodeSet.t
+  (* Removes all unreachable nodes and returns them *)
+  val gc : ('k,'e,'info,'c) t -> ('k,'e,'info,'c) t * Dag.NodeSet.t
+  val reachable : ('k,'e,'info,'c) t -> id -> Dag.NodeSet.t
 
-  (* read only dag *)
-  module Dag : Dag.S with type node = id
-  val dag : ('kind,'diff,'info) t -> ('diff,'info,id * id) Dag.t
  
-  val create_cluster : ('k,'e,'i) t -> id list -> (id * id) -> ('k,'e,'i) t
-  val cluster_of : ('k,'e,'i) t -> id -> (id * id) Dag.Cluster.t option
-  val delete_cluster : ('k,'e,'i) t -> (id * id) Dag.Cluster.t -> ('k,'e,'i) t 
-
 end
 
-module Make(OT : Map.OrderedType) : S with type id = OT.t
+module Make(OT : Map.OrderedType) : S
+with type id = OT.t
+and type Dag.node = OT.t
+and type Dag.NodeSet.t = Set.Make(OT).t
+and type Dag.NodeSet.elt = OT.t
+