Returns a list of all nodes in the graph with no dependencies.

        Returns:
            list: A list of all independent nodes.

def ind_nodes(self):
        """ Returns a list of all nodes in the graph with no dependencies.

        Returns:
            list: A list of all independent nodes.
        """
        graph = self.graph

        dependent_nodes = set(
            node for dependents
            in graph.values() for node in dependents)
        return [node_ for node_ in graph if node_ not in dependent_nodes]