diff --git a/src/lib/iter-tree-explorer.hpp b/src/lib/iter-tree-explorer.hpp
index 3cfaf7a30..17bbd8721 100644
--- a/src/lib/iter-tree-explorer.hpp
+++ b/src/lib/iter-tree-explorer.hpp
@@ -663,7 +663,7 @@ namespace lib {
         
         using FilterPredicate = typename _Traits::Functor;
         
-        FilterPredicate trafo_;
+        FilterPredicate predicate_;
         
       public:
         
@@ -672,15 +672,18 @@ namespace lib {
         
         Filter (SRC&& dataSrc, FUN&& filterFun)
           : SRC{move (dataSrc)}
-          , trafo_{forward<FUN> (filterFun)}
-          { }
+          , predicate_{forward<FUN> (filterFun)}
+          {
+            pullFilter(); // initially pull to establish the invariant
+          }
         
         
-        /** refresh state when other layers manipulate the source sequence */
+        /** refresh state when other layers manipulate the source sequence.
+         * @note possibly pulls to re-establish the invariant */
         void
         expandChildren()
           {
-            //////////////////////TODO trigger re-evaluation here...
+            pullFilter();
             SRC::expandChildren();
           }
         
@@ -689,19 +692,20 @@ namespace lib {
         bool
         checkPoint()  const
           {
-            return false; //////////////TODO filter and pull here...
+            return bool(srcIter());
           }
         
         typename SRC::reference
         yield()  const
           {
-            return *(*this);
+            return *srcIter();
           }
         
         void
         iterNext()
           {
             ++ srcIter();
+            pullFilter();
           }
         
       private:
@@ -711,11 +715,14 @@ namespace lib {
             return unConst(*this);
           }
         
-        bool
+        /** @note establishes the invariant:
+         *        whatever the source yields as current element,
+         *        has already been approved by our predicate */
+        void
         pullFilter ()
           {
-            ////////////////////////TODO while-not-filter iter
-            return false; //////////TODO return true if any result??
+            while (srcIter() and not predicate_(srcIter()))
+              ++srcIter();
           }
       };
   }
diff --git a/wiki/thinkPad.ichthyo.mm b/wiki/thinkPad.ichthyo.mm
index 1daa0c387..a963dc4d1 100644
--- a/wiki/thinkPad.ichthyo.mm
+++ b/wiki/thinkPad.ichthyo.mm
@@ -5706,12 +5706,21 @@
 <node CREATED="1511837345226" ID="ID_991593174" MODIFIED="1511837351797" TEXT="bestehende Funktor-Argument-Traits"/>
 </node>
 <node CREATED="1512523892775" ID="ID_673112014" MODIFIED="1512523896362" TEXT="Knackpunkte">
-<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1512523897422" ID="ID_1007287631" MODIFIED="1512523916092" TEXT="pullFilter">
-<icon BUILTIN="flag-yellow"/>
+<node COLOR="#338800" CREATED="1512523897422" ID="ID_1007287631" MODIFIED="1512606862773" TEXT="pullFilter">
+<icon BUILTIN="button_ok"/>
 </node>
 <node CREATED="1512523905005" ID="ID_1286045611" MODIFIED="1512523921543" TEXT="wie &quot;ausgewertet&quot; merken?">
 <icon BUILTIN="help"/>
 </node>
+<node CREATED="1512606810364" ID="ID_1956085441" MODIFIED="1512606857354" STYLE="fork" TEXT="garnicht">
+<icon BUILTIN="yes"/>
+<node CREATED="1512606816571" ID="ID_1603234787" MODIFIED="1512606853200" TEXT="mit INVARIANTE arbeiten">
+<icon BUILTIN="idea"/>
+</node>
+<node CREATED="1512606823682" ID="ID_1935971404" MODIFIED="1512606853200" TEXT="erfordert vorauseilendes Pullen">
+<icon BUILTIN="messagebox_warning"/>
+</node>
+</node>
 </node>
 </node>
 <node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1512363846804" ID="ID_1346062807" MODIFIED="1512363878326" TEXT="auto-expand">
@@ -6287,8 +6296,8 @@
 </node>
 <node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1511836579562" ID="ID_1718741335" MODIFIED="1512523829719" TEXT="verify_FilterIterator();">
 <icon BUILTIN="pencil"/>
-<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1512523719350" ID="ID_351021178" MODIFIED="1512523742972" TEXT="einfaches Pr&#xe4;dikat">
-<icon BUILTIN="flag-yellow"/>
+<node COLOR="#338800" CREATED="1512523719350" ID="ID_351021178" MODIFIED="1512606882124" TEXT="einfaches Pr&#xe4;dikat">
+<icon BUILTIN="button_ok"/>
 </node>
 <node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1512523726885" ID="ID_282579718" MODIFIED="1512523741403" TEXT="konsumieren bis leer">
 <icon BUILTIN="flag-yellow"/>