Function: matindexrank
Section: linear_algebra
C-Name: indexrank
Prototype: G
Help: matindexrank(M): gives two extraction vectors (rows and columns) for
 the matrix M such that the extracted matrix is square of maximal rank.
Doc: $M$ being a matrix of rank $r$, returns a vector with two
 \typ{VECSMALL} components $y$ and $z$ of length $r$ giving a list of rows
 and columns respectively (starting from 1) such that the extracted matrix
 obtained from these two vectors using $\tet{vecextract}(M,y,z)$ is
 invertible. The vectors $y$ and $z$ are sorted in increasing order.
