Insert a column in the QR factorization.
If [Q,R] = qr(A) is the original QR factorization of A,
then [Q,R] = qrinsert(Q,R,j,x) changes Q and R to be the
factorization of the matrix obtained by inserting an extra
column, x, before A(:,j). (If A has n columns and j = n+1,
then x is inserted after the last column of A.)
See also QR , QRDELETE , PLANEROT .