Move the series title to the correct places (depends on the system used), add series editor, and add report number (8th ed., 29.3.6.11, 29.3.7.4); and add extent of work (29.3.6.9.2)
A container-title is not a collection-title; revert part of last commit.
Create gesellschaft-fur-popularmusikforschung.csl
Update gesellschaft-fur-popularmusikforschung.csl

Now I solved the most problems, but the following 2 problems are not solved:

First: To alter „Hg. von“ into „Hg. v.“ (Under „locale“ I tried to create a new term „edited-by“, but this wasn’t useful (line 21)):

locale-edited-by.png

Second: The problem described on Nov. 04th:

R: I gave you wrong information. I want to realize the following two options (concerning bibliographic entries):

1.	Helms, Dietrich (2008). "What's the difference? Populäre Musik im System des Pop." In: PopMusicology. Perspektiven der Popmusikwissenschaft. (The latter titel is in italics!) Hg. v. Christian Bielefeldt, Udo Dahmen u. Rolf Großmann. Bielefeld: Transcript.
2.	Bielefeldt, Christian / Grossmann, Rolf / Dahmen, Udo (Hg.) (2008): PopMusicology. Perspektiven der Popmusikwissenschaft. (The latter titel is in italics!) Bielefeld: Transcript.

I have a problem with the editor names: I don't manage to divide the following two options: First: If the entry is a single book chapter or similar type, the editor names must appear after the title and in the order indicated above under (1.). Second: If the entry is the whole edited book, the editor names must appear before the title and in the order indicated above unter (2.).
Another problem is the delimiter "," in case (1.): I want to put the delimiter "u." only between the last two names. All names before must be divided with the delimiter ",".
For reports, use number instead of collection-number and also display the collection-title.
Update gesellschaft-fur-popularmusikforschung.csl
Update gesellschaft-fur-popularmusikforschung.csl
Update gesellschaft-fur-popularmusikforschung.csl

Now there remains only 1 error: (line 140) <label variable=" " form="short"/>"

The problem is: In citations I don't want to use a label in front of the page numbers!
Update gesellschaft-fur-popularmusikforschung.csl

Dear all! Thanks a lot for your comments and aid! Now there are no errors, I think! 

The only problem remaining is the "delimiter" in container titles: If the entry is a book chapter, I want to have the "u." only before the last editor name. All names before should be devided with ",". In the following examples the second version is the right one:

(1) Großmann, Rolf (2008). „Die Geburt des Pop aus dem Geist der phonographischen Reproduktion.“ In: PopMusicology: Perspektiven der Popmusikwissenschaft. Hg. v. Christian Bielefeldt u. Udo Dahmen u. Rolf Grossmann. Bielefeld: Transcript, S. 119–134.

(2) Großmann, Rolf (2008). „Die Geburt des Pop aus dem Geist der phonographischen Reproduktion.“ In: PopMusicology: Perspektiven der Popmusikwissenschaft. Hg. v. Christian Bielefeldt, Udo Dahmen u. Rolf Grossmann. Bielefeld: Transcript, S. 119–134.
Update gesellschaft-fur-popularmusikforschung.csl

My last update with no errors, I believe.
NJW (and other German law journals)

This style is based on the "Juristische Zitierweise (Stüber)" style by Oliver Wolf and Philipp Zumstein
    (https://www.zotero.org/styles/juristische-zitierweise), but the citation is adapted for the style of
    German law journals, such as NJW (Neue Juristische Wochenschrift), MMR (Multimedia und Recht) etc., as follows:
    - Citations are in-text. In German legal journals sometimes citations are used in footnotes, sometimes in-text,
      although the citation itself does not differ. With "in-text", you can choose for yourself and create the
      footnotes yourself using your word-processor if necessary.
    - Citations do not end with ".". The "." is obsolete for in-text citations. Furthermore, citations in German law
      journals are often accompanied by supplemental text and explanations.
    - Commentaries and handbooks should use publication type "entry-encyclopedia". See details below.
    - note that some law journals use slightly different citation
Changes after review

bibliography review comments have been disregarded for now. See comments in discussions.
renamed file
more changes after review
remove period after citation number and set RSC to flush insteand of margin
mastozoologia neotropical
various fixes for Harvard Cite Them Right
Renamed file
removed macro "edition-if-available" 

and other changes after review
Update gesellschaft-fur-popularmusikforschung.csl

@zuphilip and @adam3smith: I changed all options concidering your suggestions two days ago. It seems to look very good now, and the validation didn't found any errors. Thanks a lot again!