MathML test

$\begin{array}{ccccc}\hfill y& =x_0+x_1-x_2+x_3-\cdots \hfill & & \hfill & \text{by <mrow xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="\#align-eq1">1.6</mrow>}\hfill \\ \hfill & =y_0+y_1\hfill & & \hfill & \text{by (<mrow xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="\#align-eq2">1.7</mrow>)}\hfill \\ \hfill & =q_0-q_1+q_2\hfill & & \hfill & \text{by Axiom 4.}\hfill \end{array}$