intrusive

Of rocks: forced, while in a plastic or molten state, into the cavities or between the cracks or layers of other rocks.

Tending to intrude; doing that which is not welcome; interrupting or disturbing; entering without permission or welcome.

Designating a type of collection in which each item keeps track of what collection it is in, rather than the more conventional approach of a collection keeping track of what items it contains. An intrusive collection does not "own" its contents and a single item can be part of multiple intrusive collections.