Function: elltors
Section: elliptic_curves
C-Name: elltors
Prototype: G
Help: elltors(E): torsion subgroup of elliptic curve E: order, structure,
 generators.
Doc:
 if $E$ is an elliptic curve defined over a number field or a finite field,
 outputs the torsion subgroup of $E$ as a 3-component vector \kbd{[t,v1,v2]},
 where \kbd{t} is the order of the torsion group, \kbd{v1} gives the structure
 of the torsion group as a product of cyclic groups (sorted by decreasing
 order), and \kbd{v2} gives generators for these cyclic groups. $E$ must be an
 \kbd{ell} structure as output by \kbd{ellinit}.
 \bprog
 ?  E = ellinit([-1,0]);
 ?  elltors(E)
 %1 = [4, [2, 2], [[0, 0], [1, 0]]]
 @eprog\noindent
 Here, the torsion subgroup is isomorphic to $\Z/2\Z \times \Z/2\Z$, with
 generators $[0,0]$ and $[1,0]$.
