integrate the first diff verb 'ins'

--> now it becomes obvious that we've mostly
missed to integrate the Selector predicate properly
in most bindings defined thus far. Which now causes
the sub-object binding to kick in, while actually
the sub-value collection should have handled
the nested values CHILD_B and CHILD_T

src/lib/diff/tree-diff-application.hpp

@@ -176,6 +176,8 @@ namespace diff{
         {
           auto target = mutatorBinding (subject_);
           buildMutator (target);
+          TreeDiffMutatorBinding::scopeManger_ = &scopes_;
+          TreeDiffMutatorBinding::treeMutator_ = &scopes_.currentScope();
           TreeDiffMutatorBinding::initDiffApplication();
         }
     };

src/lib/diff/tree-diff-mutator-binding.cpp

@@ -236,6 +236,14 @@ namespace diff{
     
   }
   
+  void
+  TreeDiffMutatorBinding::__failMismatch (GenNode const& spec, Literal oper)
+  {
+    throw error::State(_Fmt("Unable to %s element %s. Current target binding "
+                            "did not match as expected") % oper % spec
+                      , LUMIERA_ERROR_DIFF_CONFLICT);
+  }
+  
   void
   TreeDiffMutatorBinding::__expect_end_of_scope (GenNode::ID const& idi)
   {
@@ -257,10 +265,10 @@ namespace diff{
     UNIMPLEMENTED("skip next src element and advance abstract source position");
   }
   
-  void
+  bool
   TreeDiffMutatorBinding::injectNew (GenNode const& n)
   {
-    UNIMPLEMENTED("inject a new element at current abstract position");
+    return treeMutator_->injectNew(n);
   }
   
   bool
@@ -312,7 +320,9 @@ namespace diff{
   void
   TreeDiffMutatorBinding::ins (GenNode const& n)
   {
-    injectNew (n);
+    bool success = injectNew (n);
+    if (not success)
+      __failMismatch (n, "insert");
   }
   
   void

src/lib/diff/tree-diff-mutator-binding.hpp

@@ -137,6 +137,16 @@ namespace diff{
         : scopes_()
         { }
       
+      TreeMutator&
+      currentScope()  const
+        {
+          if (0 == depth())
+            throw error::Logic("Attempt to access the current scope "
+                               "without establishing a root scope beforehand."
+                              , error::LUMIERA_ERROR_LIFECYCLE);
+          return *scopes_.top();
+        }
+      
       
       /* ==== ScopeManager interface ==== */
       
@@ -169,7 +179,6 @@ namespace diff{
         }
 
       
-      
       virtual size_t
       depth()  const
         {
@@ -261,6 +270,7 @@ namespace diff{
       void __expect_in_target  (GenNode const& elm, Literal oper);
       void __expect_further_elements (GenNode const& elm);
       void __fail_not_found (GenNode const& elm);
+      void __failMismatch (GenNode const& spec, Literal oper);
       void __expect_end_of_scope (GenNode::ID const& idi);
       void __expect_valid_parent_scope (GenNode::ID const& idi);
       
@@ -269,7 +279,7 @@ namespace diff{
       /* == Forwarding: mutation primitives == */
       
       void skipSrc();
-      void injectNew (GenNode const& n);
+      bool injectNew (GenNode const& n);
       bool matchSrc (GenNode const& n);
       bool acceptSrc (GenNode const& n);
       bool findSrc (GenNode const& n);

tests/library/diff/diff-virtualised-application-test.cpp

@@ -186,7 +186,7 @@ namespace test{
                           })
                        .buildChildMutator ([&](Opaque& target, GenNode::ID const& subID, TreeMutator::Handle buff) -> bool
                           {
-                            if (target.key_ != subID) return false; // require match on already existing child object
+                            if (target.key_ != subID) return false;    // require match on already existing child object
                             target.buildMutator (buff);               //  delegate to child to build nested TreeMutator
                             return true;
                           }))
@@ -216,6 +216,10 @@ namespace test{
                     }));
           }
         
+        /** override default size traits
+         *  to allow for sufficient buffer,
+         *  able to hold the mutator defined above.
+         */
         friend constexpr size_t
         treeMutatorSize (const Opaque*)
         {