This method is for observers to discover the structural changes that were made. This means that only elements that existed prior to the mutation (and still exist after the mutatino) need to have ElementChange records. The changes made available need not be recursive.

For example, if the an element is removed from it's parent, this method should report that the parent changed and provide an ElementChange implementation that describes the change to the parent. If the child element removed had children, these elements do not need to be reported as removed.

If an child element is insert into a parent element, the parent element should report a change. If the child element also had elements inserted into it (grandchildren to the parent) these elements need not report change.
Returns: the change information, or null if the element was not modified
Parameters:
- elem - the element