  Given an n x n matrix over the ring of differential polynomials
F(t)[\D;\delta], we show how to compute the Hermite form H of A, and a
unimodular matrix U such that UA=H. The algorithm requires a polynomial number
of operations in terms of n, deg_D(A), and deg_t(A). When F is the field of
rational numbers, it also requires time polynomial in the bit-length of the
coefficients.
