code: def quality_score(self, tests, alias=None):
          
          results = self.quality(tests, alias=alias).values()
          if results:
              return sum(results) / len(results)
          return -1
docs: Run a series of tests and return the normalized score.
              1.0:   Passed all tests.
              (0-1): Passed a fraction of tests.
              0.0:   Passed no tests.
              -1.0:  Took no tests.
  
          Args:
              tests (list): a list of functions.
              alias (dict): a dictionary mapping mnemonics to lists of mnemonics.
  
          Returns:
              float. The fraction of tests passed, or -1 for 'took no tests'.