first shot at Ticket #355: use a default-constructed "invalid" PlacementRef

Making PlacementRef default constructible this way
would resolve the immediate problems; as any access
goes through an index lookup and thus will throw.
The bool check on this special ref yields false,
so this solution seems to fill the bill.

src/proc/mobject/placement-ref.hpp

@@ -106,6 +106,13 @@ namespace mobject {
           validate(id_); 
         }
       
+      /** Default is an NIL Placement ref. It throws on any access. */
+      PlacementRef ()
+        : id_()
+        {
+          REQUIRE (!isValid(), "hash-ID clash with existing ID");
+        }
+      
       PlacementRef (PlacementRef const& r)    ///< copy ctor
         : id_(r.id_)
         { 

src/proc/mobject/session/scope-path.hpp

@@ -43,7 +43,7 @@ namespace session {
    */
   class ScopePath
     {
-      vector<RefPlacement> path_;
+      vector<Scope> path_;
       
     public:
       ScopePath();

src/proc/mobject/session/scope.cpp

@@ -42,6 +42,13 @@ namespace session {
   }
   
   
+  Scope::Scope ()
+    : anchor_()
+  {
+    REQUIRE (!anchor_.isValid());
+  }
+  
+  
   ScopeLocator::ScopeLocator()
     : focusStack_(new QueryFocusStack)
   {

src/proc/mobject/session/scope.hpp

@@ -59,6 +59,7 @@ namespace session {
       
     public:
       Scope (PlacementMO const& constitutingPlacement);
+      Scope (); ///< unlocated NIL scope
       
       static Scope const& containing (PlacementMO const& aPlacement);              //////////////TODO really returning a const& here??
       static Scope const& containing (RefPlacement const& refPlacement);