If I have boolean polynomials like $$x_1 * x_2 + x_1x_3 x_5 + x_3 + x_7x_2$$, then given variable $x_1$, how can I represent it as $$x_1 *(x_2 + x_3x_5) + x_3 + x_7x_2$$ by sage? In general, I want to find a way to represent a form of $$ax_1 + c$$, where $a$ is the linear combination of multiples of $x_1$ and $c$ is the rest polynomial.