* resurected Flagged from Eigen2Support

* reimplement .diagonal() for ProductBase to make (A*B).diagonal() more efficient!

Eigen/src/Core/Product.h

@@ -54,8 +54,6 @@ enum {
   Small = Dynamic/2
 };
 
-enum { OuterProduct, InnerProduct, UnrolledProduct, GemvProduct, GemmProduct };
-
 template<typename Lhs, typename Rhs> struct ei_product_type
 {
   typedef typename ei_cleantype<Lhs>::type _Lhs;