@Override
    public void unsafe_set(int row, int col, double val) {
        if( row == 0 ) {
            if( col == 0 ) {
                a11 = val; return;
            } else if( col == 1 ) {
                a12 = val; return;
            }
        } else if( row == 1 ) {
            if( col == 0 ) {
                a21 = val; return;
            } else if( col == 1 ) {
                a22 = val; return;
            }
        }
        throw new IllegalArgumentException("Row and/or column out of range. "+row+" "+col);
    }