Function: ellchangepointinv
Section: elliptic_curves
C-Name: ellchangepointinv
Prototype: GG
Help: ellchangepointinv(x,v): change data on point or vector of points x on an
 elliptic curve according to v=[u,r,s,t], inverse of ellchangepoint.
Doc:
 changes the coordinates of the point or vector of points $x$ using
 the inverse of the isomorphism attached to \kbd{v=[u,r,s,t]},
 i.e.~if $x'$ and $y'$ are the old coordinates, then $x=u^2x'+r$,
 $y=u^3y'+su^2x'+t$ (inverse of \kbd{ellchangepoint}).
 \bprog
 ? E0 = ellinit([1,1]); P0 = [0,1]; v = [1,2,3,4];
 ? E = ellchangecurve(E0, v);
 ? P = ellchangepoint(P0,v)
 %3 = [-2, 3]
 ? ellisoncurve(E, P)
 %4 = 1
 ? ellchangepointinv(P,v)
 %5 = [0, 1]  \\ we get back P0
 @eprog
