extensible filter: add re-evaluation (test PASS)

this is the tiny bit of operational functionality needed on top:
whenever we're reconfiguring the predicate, we need to re-trigger
the evaluation (and clear the cached value)

n.b.: I've verified in debugger that the closure is
allocated on the heap and the functors are passed by value

src/lib/itertools.hpp

@@ -366,6 +366,13 @@ namespace lib {
       typedef FilterCore<IT> _Filter;
       typedef typename _Filter::Val Val;
       
+      void
+      reEvaluate()
+        {
+          this->core_.cached_ = false;
+          this->hasData(); // re-evaluate head element
+        }
+      
     public:
       ExtensibleFilterIter() { }
       
@@ -374,6 +381,7 @@ namespace lib {
         : FilterIter<IT>(src, initialFilterPredicate)
         { }
       
+      
       template<typename COND>
       ExtensibleFilterIter&
       andFilter (COND conjunctiveClause)
@@ -385,6 +393,7 @@ namespace lib {
                         return filter(val)
                            and conjunctiveClause(val);
                       };
+          reEvaluate();
           return *this;
         }
     };