code: def _invert(self):
          
          result = defaultdict(dict)
          for test_context, src_context in six.iteritems(self.data):
              for src, lines in six.iteritems(src_context):
                  result[src][test_context] = lines
          return result
docs: Invert coverage data from {test_context: {file: line}}
          to {file: {test_context: line}}