Funding for this research was provided by:
Beijing Social Science Foundation Research Base Project (17JDGLA009)
Text and Data Mining valid from 2019-12-01
Received: 14 May 2019
Accepted: 14 November 2019
First Online: 11 December 2019
Ethics approval and consent to participate
: It is declared that the paper does not involve violation of any human rights, illegal use of data, or copyright protected materials.
: This paper does not involve any ones data (i.e., individual data, table, figures etc.).
: The authors declare that they have no competing interests.