### 2.52.5 Step by step Gauss-Jordan reduction of a matrix : `pivot`

`pivot` takes three arguments : a matrix with n rows and p columns and two integers l and c such that 0≤ l<n, 0≤ c<p and Al,c≠ 0.
`pivot(A,l,c)` performs one step of the Gauss-Jordan method using `A[l,c]` as pivot and returns an equivalent matrix with zeros in the column `c` of `A` (except at row l).
Input :

`pivot([[1,2],[3,4],[5,6]],1,1)`

Output :

`[[-2,0],[3,4],[2,0]]`

Input :

`pivot([[1,2],[3,4],[5,6]],0,1)`

Output :

`[[1,2],[2,0],[4,0]]`