In order to avoid memory leaks, environments, databases and iterators
should always be closed explicitly. Consider using call_cleanup/2 to
automate the closing/deallocation of these objects. You can always use
db_current_env/1, db_current/5 and
db_current_iterator/3 to enumerate the currently living objects.
NOTE: a database must not be closed while there are outstanding choices
for some db_fetch/3 goal that refers to that database.
Outstanding choices can be removed with a cut (!).