Text and Data Mining valid from 2015-02-17
Received: 27 October 2014
Accepted: 22 January 2015
First Online: 17 February 2015
Change Date: 24 July 2015
Change Type: Update
Change Details: The version of this Article previously published quoted an incorrect email address for Xuming Mo. Correspondence and request for materials should also be addressed to firstname.lastname@example.org. This has now been corrected in the HTML and PDF versions of the Article.
: The authors declare no competing financial interests.