doc depend directive

README.org

@@ -221,9 +221,6 @@ directives are present in the segment code the last one is retained.
   that are identical. To make use of group, elements of the output set
   have to be hashable.
 
-*** Depend directive
-
-
 *** Orphan segments
 
 By default, orphan segments have no input argument (an empty list),
@@ -234,6 +231,36 @@ If P is an instance of the pipeline object:
 P.push (segname=seg_input)
 
 
+*** Depend directive
+
+As explained in the introduction section, Pipelet offers the
+possibility to spare CPU time by saving intermediate products on disk.
+We call intermediate products the input/output data files of the
+different segments.  
+
+Each segment repository is identified by an unique key which depends
+on: 
+- the segment processing code and parameters (segment and hook
+  scripts)
+- the input data (identified from the key of the parent segments)
+
+Every change made on a segment (new parameter or new parent) will then
+give a different key, and tell the pipelet engine to compute a new
+segment instance.
+
+It is possible to add some external dependencies to the key
+computation using the depend directive: 
+
+#depend file1 file2
+
+At the very beginning of the pipeline execution, all dependencies will
+be stored, to prevent any change (code edition) between the key
+computation and actual processing.
+
+Note that this mecanism works only for segment and hook
+scripts. External dependencies are also read as the beginning of the
+pipeline execution, but only used for the key computation.
+
 *** Hierarchical data storage
 
 This system provides versionning of your data and easy access through
@@ -265,6 +292,9 @@ The storage is organized as follows:
   one of its parent's one : 
        /prefix/segname_YFLJ65/segname2_PLMBH9/
 - etc...
+  
+
+
 
 *** The segment environment