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Hi, why does this snippet produces "variable 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");

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");

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");