#include <boost/qvm/mat_operations.hpp>
namespace boost
{
    namespace qvm
    {
        //Only enabled if:
        //  is_mat<A>::value &&
        //  mat_traits<A>::cols==mat_traits<A>::rows
        template <class A>
        void set_identity( A & a );
    }
}
As if
assign( a, identity_mat< typename mat_traits<A>::scalar_type, mat_traits<A>::rows, mat_traits<A>::cols>());