use string::append to catenate strings, it could be illigel to refer to string::begin if the string is empty
use string::append to catenate strings, it could be illigel to refer to string::begin if the string is empty

cg31/tensorflow,cg31/tensorflow,cg31/tensorflow,cg31/tensorflow,cg31/tensorflow,cg31/tensorflow,cg31/tensorflow,cg31/tensorflow,cg31/tensorflow