### Variable not found

Hi, why does this snippet produces "variable 'a' not found"?

I looked all over and I just can't figure it out.

```
var('a');
var('x y z');
x = cos(a)^3;
y = sin(a)^3;
z = cos(2*a);
pl1=parametric_plot3d( (cos(t)^3, sin(t)^3, cos(2*t)), (t, 0, 2*pi));
a = pi/4;
pl2=parametric_plot3d( (x + t*(derivative(x)), y + t * (derivative(y)), z + t * (derivative(z))), (t,0,1), texture="red");
```