Function: substpol
Section: polynomials
C-Name: gsubstpol
Prototype: GGG
Help: substpol(x,y,z): in expression x, replace the polynomial y by the
 expression z, using remainder decomposition of x.
Doc: replace the ``variable'' $y$ by the argument $z$ in the ``polynomial''
 expression $x$. Every type is allowed for $x$, but the same behavior
 as \kbd{subst} above apply.

 The difference with \kbd{subst} is that $y$ is allowed to be any polynomial
 here. The substitution is done moding out all components of $x$
 (recursively) by $y - t$, where $t$ is a new free variable of lowest
 priority. Then substituting $t$ by $z$ in the resulting expression. For
 instance
 \bprog
 ? substpol(x^4 + x^2 + 1, x^2, y)
 %1 = y^2 + y + 1
 ? substpol(x^4 + x^2 + 1, x^3, y)
 %2 = x^2 + y*x + 1
 ? substpol(x^4 + x^2 + 1, (x+1)^2, y)
 %3 = (-4*y - 6)*x + (y^2 + 3*y - 3)
 @eprog
Variant: Further, \fun{GEN}{gdeflate}{GEN T, long v, long d} attempts to
 write $T(x)$ in the form $t(x^d)$, where $x=$\kbd{pol\_x}$(v)$, and returns
 \kbd{NULL} if the substitution fails (for instance in the example \kbd{\%2}
 above).