Now I've looked at Feder's lecture notes. I've appended a text of my own where in the expression for ln W in sections 1 and 3 I get the same as equation 4.6 in Feder, apart from a minus in front of one of the terms. Don't know where the mistake is.
Anyway: I'd like to know if it's possible to simplify this expression as sketched in the text. My problem is that I'm left with 1 term I can't simplify at the end of section 3. Do you know if this is done anywhere?