Section: Array Generation and Manipulations
rref is 
R = rref(A)
A default tolerance of max(size(A))*eps*norm(A,inf) is used to
detect negligible column elements.  The second form of rref 
returns a vector k as well as R
[R,k] = rref(A)
where k is a vector that correponds to the columns of A 
used as pivot columns.  If you want to control the tolerance
used to identify negligible elements, you can use the form
[R,k] = rref(A, tolerance)
This implementation of rref is based on the one from 
the matcompat lib for octave.  It is copyright Paul Kienzle,
and distributed under the GNU GPL.