docs: def retrieve_reviewers(self, product):
          """Retrieve reviewers who reviewed a given product.
  
          Args:
            product: A product specifying reviewers.
  
          Returns:
            A list of reviewers who review the product.
  
          Raises:
            TypeError: when given product isn't instance of specified product
              class when this graph is constructed.
          """
          if not isinstance(product, self._product_cls):
              raise TypeError(
                  "Type of given product isn't acceptable:", product,
                  ", expected:", self._product_cls)
          return list(self.graph.predecessors(product))