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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 add_from_existing(self, resource, timeout=-1):
 """ Adds a volume that already exists in the Storage system Args: resource (dict):
 Object to create. timeout: Timeout in seconds. Wait for task completion by default. The timeout does not abort the operation in OneView, just stop waiting for its completion. Returns: dict: Added resource. """

        uri = self.URI + "/from-existing"
        return self._client.create(resource, uri=uri, timeout=timeout)