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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def traverse_dependency_graph(self, ref, collector, memo=None):
 """Traverses module graph, starting with ref, collecting values for each ref into the sets created by the collector function. :param ref an IvyModuleRef to start traversing the ivy dependency graph :param collector a function that takes a ref and returns a new set of values to collect for that ref, which will also be updated with all the dependencies accumulated values :param memo is a dict of ref -> set that memoizes the results of each node in the graph. If provided, allows for retaining cache across calls. :returns the accumulated set for ref """


    resolved_ref = self.refs_by_unversioned_refs.get(ref.unversioned)
    if resolved_ref:
      ref = resolved_ref
    if memo is None:
      memo = dict()
    visited = set()
    return self._do_traverse_dependency_graph(ref, collector, memo, visited)