/in/foo.c: In function 'ay':
/in/foo.c:18:10: warning: implicit declaration of function 'gy' [-Wimplicit-function-declaration]
  k=y3*y4/gy(y3,y4);
          ^~
/tmp/ccf75EMw.o: In function `ay':
foo.c:(.text+0x39): undefined reference to `gy'
/tmp/ccf75EMw.o: In function `main':
foo.c:(.text.startup+0x5b): undefined reference to `gy'
collect2: error: ld returned 1 exit status