There is an error in Proposition 3.2. This
arose because the symbols considered in this section of the
paper are not exactly the same as those of section 1
(one must replace **$f_j$** in the displayed equation after 3.1 with
its transform by multiplication by -1).

In the formula of 3.2, one should consequently read **$(\nu+\mu)$** in
place of **$(\nu-\mu)$**.

Other results in the paper are not affected.