ion mapping from node name to a hashable collection
               of node names which this node has outgoing edges to.
  Returns:
    A list containing all of the node in graph in topological order.
    It is assumed that calling get_edges once for each node and caching is
    cheaper than repeatedly calling get_edges.
  Raises:
    CycleError in the event of a cycle.
  Example:
    graph = {'a': '$(b) $(c)', 'b': 'hi', 'c': '$(b)'}
    def GetEdges(node):
      return re.findall(r'\$\(([^))]\)', graph[node])
    print TopologicallySorted(graph.keys(), GetEdges)
    ==>
    ['a', 'c', b']
  c