Maxima Function
ptriangularize (M, v)
If M is a matrix with each entry a polynomial in v, return a matrix M2 such that
(1) M2 is upper triangular,
(2) M2 = E_n ... E_1 M
,
where E_1 through E_n are elementary matrices
whose entries are polynomials in v,
(3) |det (M)| = |det (M2)|
,
Note: This function doesn't check that every entry is a polynomial in v.