-String _f (char const* format, ...);
-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 = "");
+/**
+ va_list version of _f
+ */
+string v_f (char const *format, va_list args);