QR and permutation matrix

[Han]

The QR() command returns the QR factorization of a matrix and includes a permutation matrix. I cannot seem to find an example of a matrix for which the QR factorization returns a permutation matrix that is non-identity. Is anyone able to find such a case? From the looks of it, the QR() command does not appear to do any pivoting (the diagonals of R are not in non-increasing order). For example:

M2:=[[1,2],[3,5],[-1,7],[2,-1]]
QR(M2);

returns

Code:

[
  [0.258198889747,0.169657961696,0.94436252598,−0.112822554889],
  [0.774596669241,0.393298002112,−0.326894720531,−0.372097464682],
  [−0.258198889747,0.871424985073,−3.63216356146e−2,0.415490755024],
  [0.516397779494,−0.23906349148,0,0.82230285198]
],
[
  [3.87298334621,2.06559111798],
  [0,8.64484432094],
  [0,0],
  [0,0]
],
[
  [1,0,0,0],
  [0,1,0,0],
  [0,0,1,0],
  [0,0,0,1]
]

So is there no pivoting? And if not, then it appears the P matrix is superfluous.