Link and reference integrity checking

This is kind of "Delete restrict" rule, but there is no reliable way to implement this in Drupal because any module could do node_delete() without asking permission, just that. There's no hook we can intercept to validate if a node delete operation can be done or not.

Node delete operations could be intercepted by... path, using hook_form_alter(), adding/altering functionality in VBO, admin content, etc. but there may be other modules deleting nodes in some other ways we could not track.

This is a complex task, and reliable only if you can be 100% sure all delete operations are intercepted, so I decided not to spend time on that, aside that we don't needed it.

There may be partial implementations of this feature, but I would not add them here, in this module. For example, you could hide the delete button in the node form if a you find relations to the node being edited. And things of that sort... but that's not 100% sure a referred node cannot be deleted if someone uses admin content, or any other method to delete nodes.