range pack: add explicit cast to YRange Ptr

src/include/ranges/range_base.h

@@ -247,6 +247,9 @@ namespace CNORXZ
 
 	/** return YRange containig the ranges of the pack */
 	operator RangePtr() const;
+
+	/** return YRange containig the ranges of the pack */
+	operator Sptr<YRange>() const;
     };
 
     /** create a pack consisting of ranges a and b

src/lib/ranges/range_base.cc

@@ -118,7 +118,12 @@ namespace CNORXZ
     {
 	return YRangeFactory(mRs).create();
     }
-    
+
+    RangePack::operator Sptr<YRange>() const
+    {
+	return std::dynamic_pointer_cast<YRange>( YRangeFactory(mRs).create() );
+    }
+
     RangePack operator*(const RangePtr& a, const RangePtr& b)
     {
 	CXZ_ASSERT(a != nullptr, "first operand not initialized");