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