- Correspondence to : Dr Jonathan Singer, Division of Pulmonary, Critical Care, Allergy, and Sleep Medicine, Department of Medicine, University of California, San Francisco, 350 Parnassus Avenue, Suite 609, San Francisco, CA 94117, USA;
- Received September 25, 2013
- Revised November 25, 2013
- Accepted November 27, 2013
- First published December 19, 2013.
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