# subs in vector field

Hello,

```
var('a')
E.<x,y,z>=EuclideanSpace()
vf=E.vector_field([a**2,a*x,a*x*y]) # arbitrary vector field
vf.subs(a==1).display()
```

gives back the vector field without substituting the value of a

```
a^2 e_x + a*x e_y + a*x*y e_z
```

How does the funciton subs work with vector fields?

Thanks alot for your help.