<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def leaf_nodes(self):
 """ Return an interable of nodes with no edges pointing at them. This is helpful to find all nodes without dependencies. """

        # Now contains all nodes that contain dependencies.
        deps = {item for sublist in self.edges.values() for item in sublist}
        # contains all nodes *without* any dependencies (leaf nodes)
        return self.nodes - deps