Function: ellordinate
Section: elliptic_curves
C-Name: ellordinate
Prototype: GGp
Help: ellordinate(E,x): y-coordinates corresponding to x-ordinate x on
 elliptic curve E.
Doc:
 gives a 0, 1 or 2-component vector containing
 the $y$-coordinates of the points of the curve $E$ having $x$ as
 $x$-coordinate.
