/in/foo.c: In function 'main':
/in/foo.c:22:5: warning: implicit declaration of function 'gy' [-Wimplicit-function-declaration]
   j=gy(j,a[i]);
     ^~
/tmp/ccfpjywk.o: In function `main':
foo.c:(.text.startup+0x97): undefined reference to `gy'
collect2: error: ld returned 1 exit status