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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 _walk(self, target, visitor):
 """Walks the dependency graph for the given target. :param target: The target to start the walk from. :param visitor: A function that takes a target and returns `True` if its dependencies should also be visited. """

    visited = set()

    def walk(current):
      if current not in visited:
        visited.add(current)
        keep_going = visitor(current)
        if keep_going:
          for dependency in self.dependencies(current):
            walk(dependency)

    walk(target)