diff --git a/model/sync-digest.cc b/model/sync-digest.cc
index 65e3b44..863431e 100644
--- a/model/sync-digest.cc
+++ b/model/sync-digest.cc
@@ -24,6 +24,9 @@
 #include <string.h>
 
 #include "ns3/assert.h"
+#include <boost/exception/errinfo_at_line.hpp>
+
+using namespace boost;
 
 namespace ns3 {
 namespace Sync {
@@ -36,7 +39,7 @@
 
   int ok = EVP_DigestInit_ex (m_context, EVP_sha1 (), 0);
   if (!ok)
-    throw DigestCalculationError ();
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
 }
 
 Digest::~Digest ()
@@ -57,7 +60,7 @@
   int ok = EVP_DigestFinal_ex (m_context,
 			       m_buffer, &m_hashLength);
   if (!ok)
-    throw DigestCalculationError ();
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
 }
   
 std::size_t
@@ -88,6 +91,20 @@
 }
 
 
+Digest &
+Digest::operator << (const Digest &src)
+{
+  if (src.m_buffer == 0)
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
+
+  bool ok = EVP_DigestUpdate (m_context, src.m_buffer, src.m_hashLength);
+  if (!ok)
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
+  
+  return *this;
+}
+
+
 } // Sync
 } // ns3
 
diff --git a/model/sync-digest.h b/model/sync-digest.h
index 2816b3e..0030b4a 100644
--- a/model/sync-digest.h
+++ b/model/sync-digest.h
@@ -49,9 +49,6 @@
    */
   ~Digest ();
 
-  // Digest &
-  // operator << (
-
   /**
    * @brief Obtain a short version of the hash (just first sizeof(size_t) bytes
    *
@@ -68,6 +65,18 @@
   bool
   operator == (Digest &digest);
 
+  /**
+   * @brief Combine digests
+   * @param src digest to combine with
+   *
+   * The result of this combination is  hash (hash (...))
+   */
+  Digest &
+  operator << (const Digest &src);
+
+
+  
+  
 private:
   /**
    * @brief Finalize digest. All subsequent calls to "operator <<" will fire an exception