WIP: implement the recursive scope expansion

Only a Record payload constitutes a nested scope.
For all other (primitive) values, we return an empty iterator.
When used within ScopeExplorer, this implementation will just
lead to exposing any simple value once, while delving into
iteration of nested scopes

src/lib/diff/gen-node.hpp

@@ -257,7 +257,7 @@ namespace diff{
       
       
       
-      /** @internal diagnostics helper. Need to include format-helper.cpp */
+      /** @internal diagnostics helper. Include format-helper.cpp on use */
       operator string()  const
         {
           return "GenNode-"+string(idi)+"-"+string(data);
@@ -406,6 +406,8 @@ namespace diff{
   
   /**
    * Building block for monad-like depth-first expansion of a GenNode.
+   * When used within lib::IterStateWrapper, the result is an Iterator
+   * to visit the contents of a GenNodet tree recursively depth-fist.
    */
   class GenNode::ScopeExplorer
     {
@@ -439,7 +441,7 @@ namespace diff{
       iterNext (ScopeExplorer & explorer)
       {
         ScopeIter& current = explorer.scopes_.back();
-        explorer.scopes_.emplace_back(current->data.expand());
+        explorer.scopes_.emplace_back (current->data.expand());
         ++current;
         while (!explorer.scopes_.empty() && !explorer.scopes_.back())
           explorer.scopes_.pop_back();
@@ -447,6 +449,20 @@ namespace diff{
     };
   
   
+  /** @internal Core operation to expand nested scopes recursively */
+  DataCap::Locator
+  DataCap::expand()  const
+    {
+      Rec* val = unConst(this)->maybeGet<Rec>();
+      if (!val)
+        return Locator();
+      else
+        return Locator(*val);
+    }
+  
+  
+  
+  
   /**
    * Constructor for a specially crafted 'ref GenNode'.
    * The identity record of the generated object will be prepared