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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 for_target(self, target):
 """Computes and returns the PinnedJarArtifactSet that should be used to manage the given target. This returns None if the target is not a JarLibrary. :param Target target: The jar_library for which to find the managed_jar_dependencies object. :return: The the artifact set of the managed_jar_dependencies object for the target, or the default artifact set from --default-target. :rtype: PinnedJarArtifactSet """

    if not isinstance(target, JarLibrary):
      return None
    found_target = target.managed_dependencies
    if not found_target:
      return self.default_artifact_set
    return self._artifact_set_map[found_target.id]