Comment on our definitions B_p and B_z - see (6.11) on. p.217:
---------------------------------------------------------------

We should have defined these as the inverses. That is B_p should
be (B_p)^-1, and B_z should be (B_z)^{-1}. This is what
other authors do. For some reason this was changed just before
the book was printed - but we regret it now.