string _f (char const *format, ...)
__attribute__ ((format (printf, 1, 2)));
string _f (char const *format, string s, string s2 = "", string s3 = "");
string _f (char const *format, ...)
__attribute__ ((format (printf, 1, 2)));
string _f (char const *format, string s, string s2 = "", string s3 = "");