diff --git a/swh/provenance/provenance.gr.py b/swh/provenance/provenance.gr.py
index 7c135cf..d8c345e 100644
--- a/swh/provenance/provenance.gr.py
+++ b/swh/provenance/provenance.gr.py
@@ -1,139 +1,147 @@
 def revision_process_content(
-    provenance: ProvenanceInterface,
-    revision: RevisionEntry,
-    directory: DirectoryEntry
+    provenance: ProvenanceInterface, revision: RevisionEntry, directory: DirectoryEntry
 ):
     date = provenance.directory_get_date_in_isochrone_frontier(directory)
     # stack = [(directory, directory.name)]
 
-
     # XXX
     # deal with revision root (if we still want to allow directory root in D-R table)
     # should be managed in revision_add() with an "else" statment folowwing the "if"
-    # but I put it here because I need to be sure, that all directories in the outer frontier
-    # will be child of a directory (the root directory is a particular case)
-    if (date!=None) and (date<revision.date):
-        # push directory root 
+    # but I put it here because I need to be sure, that all directories in the outer
+    # frontier will be child of a directory (the root directory is a particular case)
+    if (date != None) and (date < revision.date):
+        # push directory root
         provenance.directory_add_to_revision(revision, directory, directory)
-        stack=[]
+        stack = []
     else:
         stack = [(directory, date, directory.name)]
 
     # used to store directory timestamp
-    TE={}
+    TE = {}
 
     while stack:
         dir, date, path = stack.pop()
 
         if dir.id in TE:
-            dateTE=TE[dir.id]
-            if (date==None) or (dateTE<date): 
-                date=dateTE
+            dateTE = TE[dir.id]
+            if (date == None) or (dateTE < date):
+                date = dateTE
 
         dirs = [child for child in iter(dir) if isinstance(child, DirectoryEntry)]
         dirdates = provenance.directory_get_early_dates(dirs)
 
-        nextstack=[]
+        nextstack = []
 
         # XXX look for child directory with unkown timestamp
 
         for child in dirs:
-            dateDir=dirdates.get(child.id,None)
-            dateTE=TE.get(child.dir,None)
-            if (dateTE!=None) and ((dateDir==None) or (dateTE<dateDir)) : 
-                dirdates[child.id]=dateTE
+            dateDir = dirdates.get(child.id, None)
+            dateTE = TE.get(child.dir, None)
+            if (dateTE != None) and ((dateDir == None) or (dateTE < dateDir)):
+                dirdates[child.id] = dateTE
             if dateDir is None:
                 # we gonna have to process the directory to know max(dates)
                 nextstack.append((child, None, path / child.name))
-                TE[child.id]=None
-            elif dateDir>revision.date:
+                TE[child.id] = None
+            elif dateDir > revision.date:
                 # directory seen earlier as part of the outer frontier
                 # need to be reset and manage as it was never seen before
                 nextstack.append((child, None, path / child.name))
-                TE[child.id]=None
-            elif dateDir==revision.date:
+                TE[child.id] = None
+            elif dateDir == revision.date:
                 # directory of the inner frontier
                 # nothing to do here
                 pass
-            elif dateDir<revision.date:
+            elif dateDir < revision.date:
                 # directory of the outer frontier
                 # nothing to do here
                 pass
             else:
                 # should not happen
                 print("ERROR")
 
         if nextstack:
             # we have to proceed recursively
             # we can't know max(dates)
             stack.append((dir, date, path))
-            stack+=nextstack
+            stack += nextstack
             # order in the stack is important ...
         else:
             # otherwise proceed to determine max(dates)
 
             # XXX we look for blob status
 
             blobs = [child for child in iter(dir) if isinstance(child, FileEntry)]
             blobdates = provenance.content_get_early_dates(blobs)
             for child in blobs:
-                dateBlob=blobdates.get(child.id,None)
-                if dateBlob>revision.date:
-                    # content already found 
+                dateBlob = blobdates.get(child.id, None)
+                if dateBlob > revision.date:
+                    # content already found
                     # but this revision is earliest
-                    blobdates[child.id]=None
+                    blobdates[child.id] = None
 
             # calculate max(dates)
-            if (len(blobs)+len(dirs))==0:
+            if (len(blobs) + len(dirs)) == 0:
                 # empty dir
                 # return revision.dates
-                TE[dir.id]=revision.date
+                TE[dir.id] = revision.date
                 # and we are done
             else:
-                maxdates=revision.date
+                maxdates = revision.date
                 for bdate in blobdates.values():
                     if date is not None:
-                        maxdates=max(maxdates,bdate)
+                        maxdates = max(maxdates, bdate)
                 for ddate in dirdates.values():
-                        maxdates=max(maxdates,ddate)
+                    maxdates = max(maxdates, ddate)
                 # about the directory we are processing
-                if maxdates<revision.date:
+                if maxdates < revision.date:
                     # this directory is outside the ischrone graph
                     if stack:
                         # not the root directory
                         # all directories and blobs already known
-                        TE[dir.id]=maxdates
+                        TE[dir.id] = maxdates
                     else:
                         # this is the root directory
                         provenance.directory_add_to_revision(revision, dir, path)
-                        if date==None:
-                            # should the same as provenance.directory_get_early_date(dir)==None
+                        if date == None:
+                            # should the same as
+                            # provenance.directory_get_early_date(dir)==None
                             directory_process_content(
-                                    provenance, 
-                                    directory=dir, 
-                                    relative=dir, 
-                                    prefix=PosixPath('.'))
-                        if date==None or date>maxdates: 
-                            provenance.directory_set_date_in_isochrone_frontier(dir, maxdates) # ! make sure insert makes a min
+                                provenance,
+                                directory=dir,
+                                relative=dir,
+                                prefix=PosixPath("."),
+                            )
+                        if date == None or date > maxdates:
+                            provenance.directory_set_date_in_isochrone_frontier(
+                                dir, maxdates
+                            )  # ! make sure insert makes a min
 
-                elif maxdates==revision.date:
+                elif maxdates == revision.date:
                     # the current directory is in the inner isochrone frontier
                     # that s where we can see directory nodes of the outer frontier
-                    for child in blobs: 
-                        dateBlob=blobdates.get(child.id) 
-                        if (dateBlob is None):
+                    for child in blobs:
+                        dateBlob = blobdates.get(child.id)
+                        if dateBlob is None:
                             # unkown or reset
-                            provenance.content_set_early_date(child, revision.date) 
-                            #     ! make sure it makes a min inserting it, 
+                            provenance.content_set_early_date(child, revision.date)
+                            #     ! make sure it makes a min inserting it,
                             #     if it already exists
                             provenance.content_add_to_revision(revision, child, path)
                     for child in dirs:
-                        dateDir=dirdates.get(child.id)
-                        if dateDir<revision.date:
+                        dateDir = dirdates.get(child.id)
+                        if dateDir < revision.date:
                             # this child directory is in the outer frontier
                             provenance.directory_add_to_revision(revision, child, path)
-                            if provenance.directory_get_early_date(child)==None: 
-                                # with this implementation you don't known if this is a new one or a reset
-                                directory_process_content(provenance, directory=child, relative=child, prefix=PosixPath('.'))
-                            provenance.directory_set_date_in_isochrone_frontier(child, cdate) # ! make sure insert = min
-
+                            if provenance.directory_get_early_date(child) == None:
+                                # with this implementation you don't known if this is a
+                                # new one or a reset
+                                directory_process_content(
+                                    provenance,
+                                    directory=child,
+                                    relative=child,
+                                    prefix=PosixPath("."),
+                                )
+                            provenance.directory_set_date_in_isochrone_frontier(
+                                child, cdate
+                            )  # ! make sure insert = min
diff --git a/swh/provenance/provenance.py b/swh/provenance/provenance.py
index 3a0789e..93b254e 100644
--- a/swh/provenance/provenance.py
+++ b/swh/provenance/provenance.py
@@ -1,352 +1,362 @@
 from .archive import ArchiveInterface
 from .model import DirectoryEntry, FileEntry
 from .origin import OriginEntry
 from .revision import RevisionEntry
 
 from datetime import datetime
 from pathlib import PosixPath
 from typing import Dict, List, Optional, Tuple
 
 
 class ProvenanceInterface:
     def __init__(self, **kwargs):
         raise NotImplementedError
 
     def commit(self):
         raise NotImplementedError
 
     def content_add_to_directory(
         self, directory: DirectoryEntry, blob: FileEntry, prefix: PosixPath
     ):
         raise NotImplementedError
 
     def content_add_to_revision(
         self, revision: RevisionEntry, blob: FileEntry, prefix: PosixPath
     ):
         raise NotImplementedError
 
     def content_find_first(self, blobid: str):
         raise NotImplementedError
 
     def content_find_all(self, blobid: str):
         raise NotImplementedError
 
     def content_get_early_date(self, blob: FileEntry) -> datetime:
         raise NotImplementedError
 
     def content_get_early_dates(self, blobs: List[FileEntry]) -> Dict[bytes, datetime]:
         raise NotImplementedError
 
     def content_set_early_date(self, blob: FileEntry, date: datetime):
         raise NotImplementedError
 
     def directory_add_to_revision(
         self, revision: RevisionEntry, directory: DirectoryEntry, path: PosixPath
     ):
         raise NotImplementedError
 
     def directory_get_date_in_isochrone_frontier(
         self, directory: DirectoryEntry
     ) -> datetime:
         raise NotImplementedError
 
     def directory_get_early_dates(
         self, dirs: List[DirectoryEntry]
     ) -> Dict[bytes, datetime]:
         raise NotImplementedError
 
     def directory_set_date_in_isochrone_frontier(
         self, directory: DirectoryEntry, date: datetime
     ):
         raise NotImplementedError
 
     def origin_get_id(self, origin: OriginEntry) -> int:
         raise NotImplementedError
 
     def revision_add(self, revision: RevisionEntry):
         raise NotImplementedError
 
     def revision_add_before_revision(
         self, relative: RevisionEntry, revision: RevisionEntry
     ):
         raise NotImplementedError
 
     def revision_add_to_origin(self, origin: OriginEntry, revision: RevisionEntry):
         raise NotImplementedError
 
     def revision_get_early_date(self, revision: RevisionEntry) -> datetime:
         raise NotImplementedError
 
     def revision_get_prefered_origin(self, revision: RevisionEntry) -> int:
         raise NotImplementedError
 
     def revision_in_history(self, revision: RevisionEntry) -> bool:
         raise NotImplementedError
 
     def revision_set_prefered_origin(
         self, origin: OriginEntry, revision: RevisionEntry
     ):
         raise NotImplementedError
 
     def revision_visited(self, revision: RevisionEntry) -> bool:
         raise NotImplementedError
 
 
 def directory_process_content(
     provenance: ProvenanceInterface,
     directory: DirectoryEntry,
     relative: DirectoryEntry,
     prefix: PosixPath,
 ):
     stack = [(directory, prefix)]
 
     while stack:
         current, path = stack.pop()
 
         for child in iter(current):
             if isinstance(child, FileEntry):
                 # Add content to the relative directory with the computed path.
                 provenance.content_add_to_directory(relative, child, path)
             else:
                 # Recursively walk the child directory.
                 stack.append((child, path / child.name))
 
 
 def directory_update_content(
     stack: List[Tuple[DirectoryEntry, PosixPath]],
     provenance: ProvenanceInterface,
     revision: RevisionEntry,
     directory: DirectoryEntry,
     path: PosixPath,
     subdirs: Optional[List[DirectoryEntry]] = None,
     blobs: Optional[List[FileEntry]] = None,
     blobdates: Optional[Dict[bytes, datetime]] = None,
 ):
     assert revision.date is not None
 
     # Init optional parameters if not provided.
     if subdirs is None:
         subdirs = [
             child for child in iter(directory) if isinstance(child, DirectoryEntry)
         ]
 
     if blobs is None:
         blobs = [child for child in iter(directory) if isinstance(child, FileEntry)]
 
     if blobdates is None:
         blobdates = provenance.content_get_early_dates(blobs)
 
     # Iterate over blobs updating their date if necessary.
     for blob in blobs:
         date = blobdates.get(blob.id, None)
         if date is None or revision.date < date:
             provenance.content_set_early_date(blob, revision.date)
         provenance.content_add_to_revision(revision, blob, path)
 
     # Push all subdirectories with its corresponding path to analyze them
     # recursively.
     for subdir in subdirs:
         stack.append((subdir, path / subdir.name))
 
 
 def origin_add(provenance: ProvenanceInterface, origin: OriginEntry):
     # TODO: refactor to iterate over origin visit statuses and commit only once
     # per status.
     origin.id = provenance.origin_get_id(origin)
 
     for revision in origin.revisions:
         origin_add_revision(provenance, origin, revision)
 
         # Commit after each revision
         provenance.commit()  # TODO: verify this!
 
 
 def origin_add_revision(
     provenance: ProvenanceInterface, origin: OriginEntry, revision: RevisionEntry
 ):
     stack: List[Tuple[Optional[RevisionEntry], RevisionEntry]] = [(None, revision)]
 
     while stack:
         relative, current = stack.pop()
 
         # Check if current revision has no prefered origin and update if necessary.
         prefered = provenance.revision_get_prefered_origin(current)
 
         if prefered is None:
             provenance.revision_set_prefered_origin(origin, current)
         ########################################################################
 
         if relative is None:
             # This revision is pointed directly by the origin.
             visited = provenance.revision_visited(current)
             provenance.revision_add_to_origin(origin, current)
 
             if not visited:
                 stack.append((current, current))
 
         else:
             # This revision is a parent of another one in the history of the
             # relative revision.
             for parent in iter(current):
                 visited = provenance.revision_visited(parent)
 
                 if not visited:
                     # The parent revision has never been seen before pointing
                     # directly to an origin.
                     known = provenance.revision_in_history(parent)
 
                     if known:
                         # The parent revision is already known in some other
                         # revision's history. We should point it directly to
                         # the origin and (eventually) walk its history.
                         stack.append((None, parent))
                     else:
                         # The parent revision was never seen before. We should
                         # walk its history and associate it with the same
                         # relative revision.
                         provenance.revision_add_before_revision(relative, parent)
                         stack.append((relative, parent))
                 else:
                     # The parent revision already points to an origin, so its
                     # history was properly processed before. We just need to
                     # make sure it points to the current origin as well.
                     provenance.revision_add_to_origin(origin, parent)
 
 
 def revision_add(
     provenance: ProvenanceInterface, archive: ArchiveInterface, revision: RevisionEntry
 ):
     assert revision.date is not None
     assert revision.root is not None
 
     # Processed content starting from the revision's root directory
     date = provenance.revision_get_early_date(revision)
     if date is None or revision.date < date:
         provenance.revision_add(revision)
         revision_process_content(
             provenance, revision, DirectoryEntry(archive, revision.root, PosixPath("."))
         )
 
     return provenance.commit()
 
 
 def revision_process_content(
     provenance: ProvenanceInterface, revision: RevisionEntry, directory: DirectoryEntry
 ):
     assert revision.date is not None
 
-    stack = [(directory, directory.name)]
+    # Stack of directories (and their paths) to be processed.
+    stack: List[Tuple[DirectoryEntry, PosixPath]] = [(directory, directory.name)]
+
+    # This dictionary will hold the computed dates for visited elements outside the
+    # isochrone frontier which are candidates to be added to the outer frontier (if
+    # their parent is in the inner frontier).
+    outerdirs: Dict[PosixPath, Tuple[DirectoryEntry, datetime]] = {}
 
     while stack:
-        # Get next directory to process and query its date right before
-        # processing to be sure we get the most recently updated value.
-        current, path = stack.pop()
+        # Get next directory to process and query its date right before processing to be
+        # sure we get the most recently updated value.
+        current, prefix = stack.pop()
         date = provenance.directory_get_date_in_isochrone_frontier(current)
 
         if date is None:
-            # The directory has never been seen on the isochrone graph of a
-            # revision. Its children should be checked.
+            # The directory has never been seen on the isochrone graph of a revision.
+            # Its children should be checked.
             blobs = [child for child in iter(current) if isinstance(child, FileEntry)]
-            dirs = [
+            subdirs = [
                 child for child in iter(current) if isinstance(child, DirectoryEntry)
             ]
 
-            blobdates = provenance.content_get_early_dates(blobs)
-            dirdates = provenance.directory_get_early_dates(dirs)
-
-            # Get the list of ids with no duplicates to ensure we have
-            # available dates for all the elements. This prevents takign a
-            # wrong decision when a blob occurres more than once in the same
-            # directory.
-            ids = list(dict.fromkeys([child.id for child in blobs + dirs]))
+            # Get the list of ids with no duplicates to ensure we have available dates
+            # for all the elements. This prevents taking a wrong decision when a blob
+            # occurs more than once in the same directory.
+            ids = list(dict.fromkeys([child.id for child in blobs + subdirs]))
             if ids:
-                dates = list(blobdates.values()) + list(dirdates.values())
+                # Known dates for the blobs in the current directory.
+                blobdates = provenance.content_get_early_dates(blobs)
+
+                # Known dates for the subdirectories in the current directory that
+                # belong to the outer isochrone revision of some previous processed
+                # revision.
+                subdirdates = provenance.directory_get_early_dates(subdirs)
+
+                # Known dates for the subdirectories in the current directory that are
+                # outside the isochrone frontier of the current revision.
+                outerdates = dict(
+                    [
+                        (outerdir.id, outerdate)
+                        for path, (outerdir, outerdate) in outerdirs.items()
+                        if path.parent == prefix
+                    ]
+                )
+
+                # All known dates for child nodes of the current directory.
+                dates = (
+                    list(blobdates.values())
+                    + list(subdirdates.values())
+                    + list(outerdates.values())
+                )
 
                 if len(dates) == len(ids):
                     # All child nodes of current directory are already known.
-                    maxdate = max(dates) < revision.date
+                    maxdate = max(dates)
 
                     if maxdate < revision.date:
-                        # The directory belongs to the outer isochrone frontier
-                        # of the current revision, and this is the first time
-                        # it appears as such.
-                        # TODO: maybe the parent directory is the one that has
-                        # to be added to the frontier instead!
-                        # FIXME GR : GR do not agree.
-                        #          : it means that this directory is outside the
-                        #          : isochrone frontier but may be not at the
-                        #          : frontier if its parent directory is such
-                        #          : that maxdate<revision.date
-                        #          : That's why directory of the outer isochrone
-                        #          : frontier must be filled when their parent
-                        #          : directory in the inner isochrone frontier
-                        #          : are processed.
-                        provenance.directory_set_date_in_isochrone_frontier(
-                            current, maxdate
-                        )
-                        provenance.directory_add_to_revision(revision, current, path)
-                        directory_process_content(
-                            provenance,
-                            directory=current,
-                            relative=current,
-                            prefix=PosixPath("."),
-                        )
+                        # The directory is outside the isochrone frontier of the current
+                        # revision. Is is a candidate to be added to the outer frontier.
+                        outerdirs[prefix] = (current, maxdate)
+
+                    elif maxdate == revision.date:
+                        # The current directory is inside the isochrone frontier. If any
+                        # of its children was found outside the frontier it should be
+                        # added to the outer frontier now.
+                        if outerdates:
+                            for path, (outerdir, outerdate) in outerdirs.items():
+                                if path.parent == prefix:
+                                    provenance.directory_set_date_in_isochrone_frontier(
+                                        outerdir, outerdate
+                                    )
+                                    provenance.directory_add_to_revision(
+                                        revision, outerdir, path
+                                    )
+                                    directory_process_content(
+                                        provenance,
+                                        directory=outerdir,
+                                        relative=outerdir,
+                                        prefix=PosixPath("."),
+                                    )
 
                     else:
-                        # This revision is either in the inner frontiern or out
-                        # of order. All the children from the current directory
-                        # should be analyzed (and timestamps eventually
-                        # updated) yet current.
-                        # FIXME GR 2 : I think that in this case it is more than
-                        #            : updating content timestamp. It can lead to
-                        #            : new directory node.
+                        # Either the current directory is inside the isochrone frontier
+                        # or the revision is out of order. All the children from the
+                        # current directory should be analyzed (and timestamps
+                        # eventually updated).
                         directory_update_content(
                             stack,
                             provenance,
                             revision,
                             current,
-                            path,
-                            subdirs=dirs,
+                            prefix,
+                            subdirs=subdirs,
                             blobs=blobs,
                             blobdates=blobdates,
                         )
 
                 else:
-                    # Al least one child node is unknown, ie. the directory is
-                    # not on the outer isochrone frontier of the current
-                    # revision. Its child nodes should be analyzed and current
-                    # directory updated before them.
-                    # FIXME: I believe the only different between this branche
-                    # and the two 'else' cases above is this push to the stack.
-                    # If so, we might refactor this to avoid so many branches.
-                    # stack.append((current, path))
-                    # TODO: to uncomment the above line is it necessary to
-                    # postpone the adding of dictories to the isochrone
-                    # frontier from the branch above (maxdate < revision.date).
-                    # FIXME GR 4 : if content child nodes are unkown it does matter,
-                    #            : you can process them since they will be added to
-                    #            : the content-revision table, but if a directory
-                    #            : is unkown you have to process them.
+                    # Al least one child node is unknown, ie. the directory is inside
+                    # the isochrone frontier of the current revision. Its child nodes
+                    # should be analyzed and current directory updated before them.
+                    stack.append((current, prefix))
                     directory_update_content(
                         stack,
                         provenance,
                         revision,
                         current,
-                        path,
-                        subdirs=dirs,
+                        prefix,
+                        subdirs=subdirs,
                         blobs=blobs,
                         blobdates=blobdates,
                     )
 
         elif revision.date < date:
             # The directory has already been seen on the isochrone frontier of
             # a revision, but current revision is earlier. Its children should
             # be updated.
-            directory_update_content(stack, provenance, revision, current, path)
+            directory_update_content(stack, provenance, revision, current, prefix)
             provenance.directory_set_date_in_isochrone_frontier(current, revision.date)
 
         else:
             # The directory has already been seen on the outer isochrone
             # frontier of an earlier revision. Just add it to the current
             # revision.
-            provenance.directory_add_to_revision(revision, current, path)
+            provenance.directory_add_to_revision(revision, current, prefix)