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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 _init_flow_exceptions():
 """Internal helper to initialize _flow_exceptions. This automatically adds webob.exc.HTTPException, if it can be imported. """

  global _flow_exceptions
  _flow_exceptions = ()
  add_flow_exception(datastore_errors.Rollback)
  try:
    from webob import exc
  except ImportError:
    pass
  else:
    add_flow_exception(exc.HTTPException)