Function: hyperellchangeinvert
Section: elliptic_curves
C-Name: hyperellchangeinvert
Prototype: GG
Help: hyperellchangeinvert(C,m): C being a hyperelliptic curve given by
 a Weierstrass equation, invert the change of coordinates given by m.
 C can be given either by a squarefree polynomial P such that
 C:y^2=P(x) or by a vector [P,Q] such that C:y^2+Q(x)*y=P(x) and Q^2+4P is
 squarefree.
Doc:
 $C$ being a hyperelliptic curve given by a Weierstrass equation,
 invert the change of coordinates given by $m = [e, [a,b;c,d], H]$.

 $C$ can be given either by a squarefree polynomial $P$ such that
 $C: y^{2} = P(x)$ or by a vector $[P,Q]$ such that
 $C: y^{2} + Q(x)\*y = P(x)$ and $Q^{2}+4\*P$ is squarefree.
