code: def topological_sort(dependencies, start_nodes):
      retval = []
      def edges(node): return dependencies[node][1]
      def in_degree(node): return dependencies[node][0]
      def remove_incoming(node): dependencies[node][0] = in_degree(node) - 1
      while start_nodes:
          node = start_nodes.pop()
          retval.append(node)
          for child in edges(node):
              remove_incoming(child)
              if not in_degree(child):
                  start_nodes.append(child)
      leftover_nodes = [node for node in list(dependencies.keys())
                        if in_degree(node) > 0]
      if leftover_nodes:
          raise CyclicDependency(leftover_nodes)
      else:
          return retval