This procedure evaluates $a\left(x\right)$ over $\frac{{\mathrm{Zp}}_x}{b\left(x\right)}$.

$\mathrm{modpol}\left(x^2-1\,x\,x\,7\right)$

$6$

$\mathrm{modpol}\left(x^8+2x\,x^3-9x+2\,x\,3\right)$

$x^2+2x$