/in/foo.c: In function 'main':
/in/foo.c:8:7: warning: implicit declaration of function 'F' [-Wimplicit-function-declaration]
   s=s+F(i);
       ^
/in/foo.c: In function 'F':
/in/foo.c:19:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^