Value of the field 'hierarchy' in the table 'vocabulary' is not being set correctly when terms are added or deleted.

* Take a simple vocabulary with a few terms.
* Add a new term.

The value of 'hierarchy' in the table 'vocabulary' is incorrectly set to '1'. It should be set to '0' as per:

http://api.drupal.org/api/drupal/modules--taxonomy--taxonomy.module/func...

where it says:

"A hierarchy with no parents in any of its terms will be given a hierarchy of 0. If terms contain at most a single parent, the vocabulary will be given a hierarchy of 1. If any term contain multiple parents, the vocabulary will be given a hieararchy of 2."

* Now drag a term to the right which means it now has a parent and save.
* 'hierarchy' value will stay at '1'.
* Drag the term back to the left so that all terms are at the same base level and click save.
* 'hierarchy' value will now correctly change to '0'.

Furthermore,

* Delete a term.
(the function taxonomy_check_vocabulary_hierarchy() is called)
* 'hierarchy' value will now INCORRECTLY change to '1'.

So adding a term or deleting a term will wrongly set the 'hierarchy' value to '1'. But it looks like taxonomy_check_vocabulary_hierarchy() is only called when deleting a term.

My gut feeling is that the fault is in taxonomy_check_vocabulary_hierarchy() somewhere around the code

    if ($term->tid == $changed_term['tid']) {
      $term = (object) $changed_term;
      $term->parents = $term->parent;
    }

My other feeling is that some devs somewhere have the wrong idea that a flat hierarchy should mean a value of '1'. This is wrong; 0 means flat, 1 means one or more terms have parents, 2 means some terms have more than one parent.