sub_topics - Subtopics for hierarchical definition

Using subtopics a chapter hierarchy can be established. Because subtopics are topics again the nesting level for topics is not limited. However, it is suggested not to use more than two or three levels.

The title structure is the same for all languages, i.e. independent on the language the title has the same number of subtopics. The subtopics, however, can be provided in several languages again.