diff --git a/Trajectories/trajectories.py b/Trajectories/trajectories.py
index b4abed16c0595e04dbc5970a8a145f60dc9c261d..7742cf5cfdf9812729517943b5b7517b65639954 100755
--- a/Trajectories/trajectories.py
+++ b/Trajectories/trajectories.py
@@ -96,6 +96,8 @@ expanded_traj = [] # an element of expanded_traj is a trajectory,
                    # represented by a list of instantaneous eddies
 
 if "cost_function" in g.edge_properties:
+    # Build my_subgraphs, a dictionary of nodes at each date:
+
     my_subgraphs = {}
 
     for v in g.vertices():
@@ -109,6 +111,8 @@ if "cost_function" in g.edge_properties:
             my_subgraphs[date_index] = [int(v)]
 
     for date_index in sorted(my_subgraphs):
+        # Consider all outgoing edges at a given date:
+
         edges = []
         all_cost = []