Function: ellchangeinvert
Section: elliptic_curves
C-Name: ellchangeinvert
Prototype: GG
Help: ellchangeinvert(E,v): inverts the transformations given
 by v=[u,r,s,t].
Doc:
 Invert the transformations given by \kbd{v=[u,r,s,t]} and return
 the transformation \kbd{[1/u,-r/u\^{}2,-s/u,(s*r-t)/u\^{}3]}.
 \bprog
 ? v = [u,r,s,t]; P = [x,y]; E = ellinit([1,1]);
 ? w = ellchangeinvert(E, v)
 %2 = [1/u,-r/u^2,-s/u,(s*r-t)/u^3]
 ? ellchangepointinv(E,P,v) == ellchangepoint(E,P,w)
 %3 = 1
 @eprog\noindent Note that the elliptic curve $E$ is not needed in itself,
 it indicates the domain over which the transformation is defined. This
 is required when $E$ and the transformation are defined over a number field,
 in order to support number field elements given in column form.
