@brief User function type to use with bt_value_map_foreach().
\p object is a <em>weak reference</em>: you \em must pass it to bt_get()
-if you need your own reference.
+if you need to keep a reference after this function returns.
This function \em must return \c true to continue the map value object
traversal, or \c false to break it.
map value object \p map_obj.
The value object passed to the user function is a <b>weak reference</b>:
-you \em must pass it to bt_get() if you need your own reference.
+you \em must pass it to bt_get() if you need to keep a persistent
+reference after the user function returns.
The key passed to the user function is only valid in the scope of
this user function call.