*
* For API usage, in general,
* - any function _modifying_ the tree or tags (inserting or deleting
*
* For API usage, in general,
* - any function _modifying_ the tree or tags (inserting or deleting
* exclude any functions reading the tree.
* - any function _reading_ the tree or tags (looking up items or tags,
* gang lookups) must exclude modifications to the tree, but may occur
* exclude any functions reading the tree.
* - any function _reading_ the tree or tags (looking up items or tags,
* gang lookups) must exclude modifications to the tree, but may occur