diff --git a/src/lib/iter-tree-explorer.hpp b/src/lib/iter-tree-explorer.hpp
index 49041c0c3..ce35330b0 100644
--- a/src/lib/iter-tree-explorer.hpp
+++ b/src/lib/iter-tree-explorer.hpp
@@ -487,6 +487,8 @@ namespace lib {
         BaseAdapter(SRC && src)     : SRC(src) { }
         
         void expandChildren() { }
+        size_t depth()  const { return 0; }
+
       };
     
     
@@ -512,7 +514,7 @@ namespace lib {
      * @tparam SRC the wrapped source iterator, typically a TreeExplorer or nested decorator.
      * @tparam FUN the concrete type of the functor passed. Will be dissected to find the signature
      */
-    template<class SRC, class FUN>
+    template<class SRC, class FUN, bool expandToLeaf =false>
     class Expander
       : public SRC
       {
@@ -535,7 +537,9 @@ namespace lib {
           : SRC{move (parentExplorer)}                           // NOTE: slicing move to strip TreeExplorer (Builder)
           , expandChildren_{forward<FUN> (expandFunctor)}
           , expansions_{}
-          { }
+          {
+            maybeExpandToExhaustion();
+          }
         
         
         /** core operation: expand current head element */
@@ -551,6 +555,22 @@ namespace lib {
               expansions_.push (move(expanded));
           }
         
+        void
+        maybeExpandToExhaustion()
+          {
+            if (not (expandToLeaf and this->checkPoint())) return;
+            
+            while (true)
+              {
+                ResIter expanded{ 0 < depth()? expandChildren_(*expansions_)
+                                             : expandChildren_(*this)};
+                if (isnil(expanded))
+                  break;
+                iterate();     // consume current head element
+                expansions_.push (move(expanded));
+              }
+          }
+        
         /** diagnostics: current level of nested child expansion */
         size_t
         depth()  const
@@ -577,6 +597,13 @@ namespace lib {
         
         void
         iterNext()
+          {
+            iterate();
+            maybeExpandToExhaustion();
+          }
+        
+        void
+        iterate()
           {
             if (0 < depth())
               {
@@ -595,7 +622,7 @@ namespace lib {
     class AutoExpander
       : public SRC
       {
-        static_assert(is_StateCore<SRC>::value, "need wrapped state as predecessor in pipeline");
+        static_assert(is_StateCore<SRC>::value, "need wrapped state core as predecessor in pipeline");
         
       public:
         /** pass through ctor */
@@ -980,6 +1007,19 @@ namespace lib {
         }
       
       
+      /** @todo WIP 12/17 auto expand down to the leafs
+       */
+      template<class FUN>
+      auto
+      expandLeaf (FUN&& expandFunctor)
+        {
+          using ResCore = iter_explorer::Expander<SRC, FUN, true>;
+          using ResIter = typename _DecoratorTraits<ResCore>::SrcIter;
+          
+          return TreeExplorer<ResIter> (ResCore {move(*this), forward<FUN>(expandFunctor)});
+        }
+      
+      
       /** @todo WIP 12/17 auto expand all
        */
       auto
@@ -992,16 +1032,6 @@ namespace lib {
         }
       
       
-      /** @todo WIP 12/17 auto expand down to the leafs
-       */
-      auto
-      expandLeaf()
-        {
-          UNIMPLEMENTED ("depth-first expand immediately, until reaching the leaf elements");
-          return *this;
-        }
-      
-      
       /** adapt this TreeExplorer to pipe each result value through a transformation function.
        * Several "layers" of mapping can be piled on top of each other, possibly mixed with the
        * other types of adaptation, like the child-expanding operation, or a filter. Obviously,
diff --git a/tests/library/iter-tree-explorer-test.cpp b/tests/library/iter-tree-explorer-test.cpp
index 96597c831..da6b841f4 100644
--- a/tests/library/iter-tree-explorer-test.cpp
+++ b/tests/library/iter-tree-explorer-test.cpp
@@ -822,19 +822,19 @@ namespace test{
                                   {
                                     uint val = get<0>(tup);
                                     uint sum = get<1>(tup);
-                                    return singleValIterator (Tu2{val-1, sum+val});
+                                    return val? singleValIterator (Tu2{val-1, sum+val})
+                                              : SingleValIter<Tu2>();
                                   };
           
-#if false /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #888
           cout << materialise(
                     treeExplore(CountDown{4})
                       .transform([](uint i){ return Tu2{i,0}; })
-//                    .expand(summingExpander)                      //////////////////TODO why isn't that result iterator (singleValIterator<Tu2>) accepted? It should be
-                      .expandLeaf()
+                      .expandLeaf(summingExpander)
                       .transform([](Tu2 res){ return get<1>(res); })
                   ) <<endl;
-#endif    /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #888
           
+#if false /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #888
+#endif    /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #888
         }
       
       
diff --git a/wiki/thinkPad.ichthyo.mm b/wiki/thinkPad.ichthyo.mm
index 7cd94d9c1..e62861921 100644
--- a/wiki/thinkPad.ichthyo.mm
+++ b/wiki/thinkPad.ichthyo.mm
@@ -5774,6 +5774,28 @@
 <node CREATED="1512844769176" ID="ID_1306087014" MODIFIED="1512844774891" TEXT="expandChildren eager"/>
 <node CREATED="1512844775893" ID="ID_781402390" MODIFIED="1512844784866" TEXT="sofort erstes Kind liefern"/>
 <node CREATED="1512844785526" ID="ID_244222856" MODIFIED="1512844788897" TEXT="rekursiv"/>
+<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1512872529062" ID="ID_1993110543" MODIFIED="1512872543139" TEXT="geht nicht einfach">
+<icon BUILTIN="messagebox_warning"/>
+<node CREATED="1512872546187" ID="ID_633024773" MODIFIED="1512872616266" TEXT="Problem: Blatt erkennen">
+<node CREATED="1512872574000" ID="ID_262264527" MODIFIED="1512872587090" TEXT="m&#xfc;&#xdf;te auf dem letzten Ergebnis stehen bleiben"/>
+<node CREATED="1512872560761" ID="ID_1013659640" MODIFIED="1512872573339" TEXT="expandChildren() geht einfach weiter"/>
+</node>
+<node CREATED="1512872618801" ID="ID_1769639916" MODIFIED="1512872632495" TEXT="nur in Expander selber implementierbar">
+<icon BUILTIN="idea"/>
+<node CREATED="1512872646574" ID="ID_419478495" MODIFIED="1512872658944" TEXT="eager feed"/>
+<node CREATED="1512872676162" ID="ID_1452543628" MODIFIED="1512872683205" TEXT="sofort weiter expanden"/>
+<node CREATED="1512872684337" ID="ID_90684575" MODIFIED="1512872695451" TEXT="aber am Ende nicht weiterschalten"/>
+</node>
+<node CREATED="1512875104589" ID="ID_477086316" MODIFIED="1512875117813" TEXT="Frage: sinnvoll?">
+<icon BUILTIN="help"/>
+<node COLOR="#338800" CREATED="1512875141521" ID="ID_152225336" MODIFIED="1512875149513" TEXT="versuchshalber mal eingebaut">
+<icon BUILTIN="button_ok"/>
+</node>
+<node BACKGROUND_COLOR="#fdfdcf" COLOR="#ff0000" CREATED="1512875161638" ID="ID_681111567" MODIFIED="1512875181807" TEXT="ist es die zus&#xe4;tzliche Komplexit&#xe4;t wert?">
+<icon BUILTIN="help"/>
+</node>
+</node>
+</node>
 </node>
 </node>
 <node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1512363286920" ID="ID_1420123769" MODIFIED="1512363305422" TEXT="Zusammenspiel">