docs: def _list_samples(self, predicate=None):
          """List all samples that meet the predicate or all if predicate is not specified.
  
          Args:
              predicate: Match samples against this predicate (or all if not specified)
  
          Returns:
              List of the md5s for the matching samples
          """
          cursor = self.database[self.sample_collection].find(predicate, {'_id':0, 'md5':1})
          return [item['md5'] for item in cursor]