### 2.42.20 Exchange two rows : `rowSwap`

`rowSwap` takes three arguments : a matrix *A* and two integers
*n*1 and *n*2.

`rowSwap` returns the matrix obtained by exchanging in *A*, the row of
index *n*1 with the row of index *n*2.

Input :

`rowSwap([[1,2],[3,4]],0,1)`

Output :

`[[3,4],[1,2]]`