<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def walk(self, walk_func):
 """ Walks each node of the graph in reverse topological order. This can be used to perform a set of operations, where the next operation depends on the previous operation. It's important to note that walking happens serially, and is not paralellized. Args: walk_func (:class:`types.FunctionType`):
 The function to be called on each node of the graph. """

        nodes = self.topological_sort()
        # Reverse so we start with nodes that have no dependencies.
        nodes.reverse()

        for n in nodes:
            walk_func(n)