library function to allow users to put collections of things into other collections?

Is there any already written library function to allow users to put collections of things into other collections?

Similar to how Google docs lets users create different docs, then group them within a collection, but also lets users drag collections into other collections.

I'd like to do that with user-created government agency name categories.  

For example, I let user create some agency names: Air Force, Marines, Navy, Army.  Then I let them create an umbrella collection called "Pentagon", and let users drag Air Force, Marines, Navy, etc. into the umbrella collection.

What is complicating this, is that unlike the Google docs example where a doc is a discrete single object, user may discover smaller sub-sets of Army, and wish to categorize those within the Army name, which is not yet a collection.

Other example: user would like to add a new collection "Office of the President" and put OMB and Pentagon under that as equals.

Each user should be able to create their own collections so that multiple government department name categories exist.  This is so that they can call their own categories that may be of a different scope than what other users need.

What would the data model look like for this?

I'm hoping someone else has worked on this problem, probably not with government agency names, but perhaps the same principle with other objects.

Thanks!

Alex Glaros

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