All link types (link to Entry) should be accessible in both directions. So I can create a one-to-many connection and either access (and aggregate) the many entries from the one. Or I can get the one Entry from each of the many Entries.

Example:
I have the Libraries "Projects", "Stories" and "Tasks".
A Project can have many Stories and a Story can have many Tasks.
If I want to have a condition that any Story must have exactly one Project and any Task exactly one Story and want a Calculation field on the Projects Library that Lists all Tasks in that Project I currently have to double link all the Libraries.

Projects (1) <-> (n) Stories (1) <-> (n) Tasks

Get all Tasks
P.getAllStories().getAllTasks()

Ensure set parrent
T.hasStory()
S.hasProject()