Function: ellchangecurve
Section: elliptic_curves
C-Name: ellchangecurve
Prototype: GG
Help: ellchangecurve(E,v): change data on elliptic curve according to
 v=[u,r,s,t].
Description:
 (gen, gen):ell        ellchangecurve($1, $2)
Doc:
 changes the data for the elliptic curve $E$
 by changing the coordinates using the vector \kbd{v=[u,r,s,t]}, i.e.~if $x'$
 and $y'$ are the new coordinates, then $x=u^2x'+r$, $y=u^3y'+su^2x'+t$.
 $E$ must be an \kbd{ell} structure as output by \kbd{ellinit}. The special
 case $v = 1$ is also used instead of $[1,0,0,0]$ to denote the
 trivial coordinate change.
