The curve associated to the given genus one model.
In the degree 2 case the curve is hyperelliptic
of the form y2 + f(x, z) y - g(x, z) = 0,
but is only created explicitly as a hyperelliptic curve
when HyperellipticCurve is called;
otherwise it is created as a general curve in a weighted projective space
in which the variables x, z, and y have weights 1, 1, and 2).
In the degree 4 case the curve is an intersection
of two quadrics in P3.
An error results in degenerate cases where the equations of the model
do not define a curve