code: def list_database(db=None):
      
      if db is None:
          return CONNECTION.get_connection().database_names()
      return CONNECTION.get_connection()[db].collection_names()
docs: Lists the names of either the databases on the machine or the collections
      of a particular database
      
      @param db: the database for which to list the collection names;
          if db is None, then it lists all databases instead
          the contents of the database with the name passed in db