diff --git a/src/lib/diff/diff-message.hpp b/src/lib/diff/diff-message.hpp
index 047ad9d52..4538c9c4f 100644
--- a/src/lib/diff/diff-message.hpp
+++ b/src/lib/diff/diff-message.hpp
@@ -157,11 +157,14 @@ namespace diff{
        *          this operation needs to impose a _side effect:_ it materialises
        *          the complete diff sequence at once into a heap allocated buffer.
        */
-      DiffMessage& withDiagnostics();
+      DiffMessage& updateDiagnostics();
     };
   
-  namespace {
+  
+  
+  namespace { // Implementation: take snapshot to enable diagnostics
     
+    /** "materialised view" of the diff sequence */
     struct DiffSnapshot
       : std::vector<DiffStep>
       {
@@ -176,6 +179,15 @@ namespace diff{
     using _RangeIT = RangeIter<_VecIter>;
     using _Wrapped = WrappedLumieraIter<_RangeIT>;
     
+    /**
+     * Decorator to be layered transparently on top of DiffMessage.
+     * Actually, what we do is to discharge the diff generator into
+     * the DiffSnapshot buffer and then replace the link to the original
+     * generator to this decorator, which, when iterated, yields the
+     * contents of the DiffSnapshot one by one. But since all DiffSteps
+     * are now stored into that DiffSnapshot _buffer we control,_ we're
+     * able to produce a diagnostic listing of the complete sequence.
+     */
     class MaterialisedDiffMessageBuffer
       : private DiffSnapshot
       , public _Wrapped
@@ -194,8 +206,18 @@ namespace diff{
       };
   }
   
+  /** @par operational semantics
+   * Since the underlying generator of the DiffStep sequence is an iterator,
+   * the "materialised view" can only capture what's left at the point when
+   * updateDiagnostics() is invoked. The captured rest sequence seamlessly
+   * becomes the new generator and the old generator object is released,
+   * since the assignment of the new backend typically removes the last
+   * reference in the smart-ptr managing the generation backend. This
+   * process can be repeated and then the [diagnostics](operator string())
+   * will show the remainder of the sequence _left at that point._
+   */
   inline DiffMessage&
-  DiffMessage::withDiagnostics()
+  DiffMessage::updateDiagnostics()
   {
     return *this = DiffMessage{new MaterialisedDiffMessageBuffer(*this)};
   }
diff --git a/src/lib/format-util.hpp b/src/lib/format-util.hpp
index f9d8ac90f..56a68ebcc 100644
--- a/src/lib/format-util.hpp
+++ b/src/lib/format-util.hpp
@@ -205,6 +205,13 @@ namespace util {
     return buffer.str().substr(0, len - delim.length());
   }
   
+  template<class X>
+  inline string
+  join (std::initializer_list<X> const&& ili, string const& delim =", ")
+  {
+    return join (ili, delim);
+  }
+  
   
 } // namespace util
 #endif /*LIB_FORMAT_UTIL_H*/
diff --git a/tests/library/diff/diff-message-test.cpp b/tests/library/diff/diff-message-test.cpp
index 2dfe3abf4..a63d4f8a0 100644
--- a/tests/library/diff/diff-message-test.cpp
+++ b/tests/library/diff/diff-message-test.cpp
@@ -33,7 +33,6 @@
 #include "lib/iter-adapter-stl.hpp"
 #include "lib/time/timevalue.hpp"
 #include "lib/format-util.hpp"
-#include "lib/format-cout.hpp" ///////////////TODO remove when done
 #include "lib/util.hpp"
 
 #include <string>
@@ -42,6 +41,7 @@
 using lumiera::error::LUMIERA_ERROR_ITER_EXHAUST;
 using lib::iter_stl::IterSnapshot;
 using lib::iter_stl::snapshot;
+using util::contains;
 using util::isnil;
 using util::join;
 using std::string;
@@ -294,16 +294,35 @@ namespace test{
                              ,set(ATTRIB1)
                              ,del(CHILD_T)};
           
-          cout << diffMsg <<endl;
+          // initially only the default diagnostics of IterSource is shown (rendering the element type)
+          CHECK (string(diffMsg) == "IterSource<DiffLanguage<TreeDiffInterpreter, GenNode>::DiffStep>");
           
-          diffMsg.withDiagnostics();
-          cout << diffMsg <<endl;
+          // transparently take a snapshot
+          diffMsg.updateDiagnostics();
           
+          // now the whole sequence is rendered explicitly
+          string expectedRendering = join ({ins(TYPE_X), set(ATTRIB1), del(CHILD_T)});
+          CHECK (contains (string(diffMsg), expectedRendering));
+          
+          CHECK (string(set(ATTRIB1)) == "set(GenNode-ID(\"α\")-DataCap|«int»|1)");
+          
+          // and we can still iterate...
           CHECK (!isnil (diffMsg));
           CHECK (ins(TYPE_X)  == *diffMsg);
           CHECK (set(ATTRIB1) == *++diffMsg);
+          
+          // NOTE: in fact only the remainder of the sequence is captured...
+          diffMsg.updateDiagnostics();
+          CHECK (not contains (string(diffMsg), string(ins(TYPE_X))));
+          CHECK (    contains (string(diffMsg), string(set(ATTRIB1))));
+          CHECK (    contains (string(diffMsg), string(del(CHILD_T))));
+          
+          // and we can still continue to iterate...
           CHECK (del(CHILD_T) == *++diffMsg);
           CHECK (isnil (++diffMsg));
+          
+          diffMsg.updateDiagnostics();
+          CHECK (string(diffMsg) == "Diff--{}");
         }