namespace boost
{
    namespace qvm
    {
        //Only enabled if:
        //  is_mat<A>::value
        template <class A>
        typename mat_traits<A>::scalar_type &
        mat_index_write( A & a, int r, int c );
    }
}

Returns:

A mutable reference to the element of a at row r and column c.

Preconditions:

r >= 0 && r < mat_traits<A>::rows && c >= 0 && c < mat_traits<A>::cols.

Note:

This function uses template metaprogramming and can be used even if the optional access function mat_traits<A>::write_element_idx is not available.