Extract the ProcessArbitraryContent method.
Rename the method determining if a genex accepts arbitrary content.

The meaning of this will be expanded to generator expressions with
more than a single parameter.
Make it possible for any genex to have arbitrary content at the end.
Add the JOIN generator expression.

This generator expression joins a list with a separator. The separator
may contain arbitrary content, such as commas, which is ordinarily a
delimiter in the generator expression syntax.
Merge topic 'join-genex'

a7ba452 Add the JOIN generator expression.
96ec314 Make it possible for any genex to have arbitrary content at the end.
bd638ee Rename the method determining if a genex accepts arbitrary content.
dc742fe Extract the ProcessArbitraryContent method.