Is coeff even necessary given the simplicity of the arbitrary point?

```
>>> var('a:d t');Ellipse((a,b),c,d).arbitrary_point(t)
(a, b, c, d, t)
Point(a + c*cos(t), b + d*sin(t))

a1,a2,a3 = 0,self.hradius,self.center.x
b1,b2,b3 = self.vradius,0,self.center.y
```