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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 force_invalidate(self, vts):
 """Force invalidation of a VersionedTargetSet."""

    for vt in vts.versioned_targets:
      self._invalidator.force_invalidate(vt.cache_key)
      vt.valid = False
    self._invalidator.force_invalidate(vts.cache_key)
    vts.valid = False