diff --git a/src/lib/hierarchy-orientation-indicator.hpp b/src/lib/hierarchy-orientation-indicator.hpp
index 826204b61..d02ee997e 100644
--- a/src/lib/hierarchy-orientation-indicator.hpp
+++ b/src/lib/hierarchy-orientation-indicator.hpp
@@ -43,6 +43,8 @@
 #define HIERARCHY_ORIENTATION_INDICATOR_H
 
 
+#include <cstddef>
+#include <limits>
 #include <string>
 
 namespace lib {
@@ -52,39 +54,44 @@ namespace lib {
   
   class OrientationIndicator
     {
-      string nothing_;
-
+      size_t refLevel_;
+      ptrdiff_t offset_;
+      
     public:
       OrientationIndicator()
-        : nothing_ ()
-        {
-        }
+        : refLevel_(0)
+        , offset_(0)
+        { }
       
       // using default copy/assignment
       
-      operator int()  const
+      operator ptrdiff_t()  const
         {
-          UNIMPLEMENTED ("tbw");
+          return offset_;
         }
       
-      /* == X interface for == */
-
+      
+      
       void
       markRefLevel (size_t newRefLevel)
         {
-          UNIMPLEMENTED ("tbw");
+          REQUIRE (newRefLevel < size_t(std::numeric_limits<ptrdiff_t>::max()) );
+          
+          offset_ = newRefLevel - refLevel_;
+          refLevel_ = newRefLevel;
         }
       
       OrientationIndicator&
       operator+= (int adj)
         {
-          UNIMPLEMENTED ("tbw");
+          offset_ += adj;
+          return *this;
         }
       
       OrientationIndicator&
       operator-= (int adj)
         {
-          this->operator +=(-adj);
+          offset_ -= adj;
           return *this;
         }
       
diff --git a/tests/library/hierarchy-orientation-indicator-test.cpp b/tests/library/hierarchy-orientation-indicator-test.cpp
index a20f03c85..bdad1fa49 100644
--- a/tests/library/hierarchy-orientation-indicator-test.cpp
+++ b/tests/library/hierarchy-orientation-indicator-test.cpp
@@ -326,6 +326,9 @@ namespace test {
   /***************************************************************************
    * @test describing and rebuilding a tree structure
    *       while visiting the tree in depth first order.
+   *       To keep track of the level changes during that navigation,
+   *       we use an indicator to represent the relative level difference
+   *       compared to the previously visited node in the tree.
    *       
    * @see HierarchyOrientationIndicator
    * @see DispatcherInterface_test
@@ -375,7 +378,7 @@ namespace test {
           CHECK (-1 == orient);
           
           orient.markRefLevel (2);
-          CHECK (+3 == orient);
+          CHECK (+2 == orient);
           
           orient.markRefLevel (2);
           CHECK ( 0 == orient);
@@ -392,15 +395,16 @@ namespace test {
           
           orient += 200;
           orient -= 190;
-          CHECK (+13 == orient);
+          CHECK (+7 == orient);
           
           OrientationIndicator o2(orient);
           o2.markRefLevel(0);
-          CHECK (+3 == o2);
-          CHECK (+13 == orient);
+          CHECK (-2 == o2);
+          CHECK (+7 == orient);
         }
     };
   
+  
   /** Register this test class... */
   LAUNCHER(HierarchyOrientationIndicator_test, "unit common");