Text and Data Mining valid from 2014-07-03
Received: 28 March 2014
Accepted: 11 June 2014
First Online: 3 July 2014
: Yes, there is potential Competing Interest. The authors H.H., J.W. and S.N. are employees of NTT Communication Science Laboratories, which is a basic-science research section of Nippon Telegraph and Telephone Corporation.