How to get an echelon form of a matrix on any given column indices?

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Dear all,

Let $k< n$. Given a random $k\times n$ matrix $G$ of rank $k$. The echelon form of $G$ returned by G.echelon_form() has a $k\times k$ identity matrix on the first $k$ columns.

How to get the echelon form of $G$ such that the echelon form has a $k\times k$ identity matrix on $\mathbf{any}$ $\mathbf{given}$ $k$ $\mathbf{columns}$ of G? Such as the last $k$ columns.

Thanks!