# How to permute strings from two lists (or vectors)

Suppose I have to vector of variables

```
NB=vector(var('x', n=3, latex_name='x'))
B=vector(var('y', n=4, latex_name='ϵ'))
```

I would like to construct a function say `BNB(vec1, vec2, pr, pc)`

such that when applied to `B`

and for legitiamte values --- i.e. : here, `NB`

should be less than 3 and `B`

should be less than 4 ---, permute the correspondant $x$ and $\epsilon from a vector to an other. That is :

`BNB(B, NB, 1,2)`

gives

$B=[\epsilon_0,x_2,\epsilon_3,\epsilon_4]$ and $NB=[x_0,x_1,\epsilon_1,x_3]$

By advance thanks for the help.