db_sync(:What)- reload
 - Database is reloaded from file if the file was modified since loaded.
 - update
 - As 
reload, but use incremental loading if possible. This allows for two processes to examine the same database file, where one writes the database and the other periodycally callsdb_sync(update)to follow the modified data. - gc
 - Database was re-written, deleting all retractall statements. This is the 
same as 
gc(50). - gc(Percentage)
 - GC DB if the number of deleted terms is the given percentage of the total number of terms.
 - close
 - Database stream was closed
 - detach
 - Remove all registered persistency for the calling module
 - nop
 - No-operation performed
 
With unbound What, db_sync/1 reloads the database if it was modified on disk, gc it if it is dirty and close it if it is opened.