/in/foo.c: In function 'main':
/in/foo.c:12:2: warning: implicit declaration of function 'Output' [-Wimplicit-function-declaration]
  Output(a,na);
  ^~~~~~
/in/foo.c:14:7: warning: implicit declaration of function 'UnionSet' [-Wimplicit-function-declaration]
  nc = UnionSet(a,na, b,nb, c);
       ^~~~~~~~
/tmp/ccrhU9Vg.o: In function `main':
foo.c:(.text.startup+0xab): undefined reference to `Output'
foo.c:(.text.startup+0xba): undefined reference to `Output'
foo.c:(.text.startup+0xd4): undefined reference to `UnionSet'
foo.c:(.text.startup+0xe0): undefined reference to `Output'
collect2: error: ld returned 1 exit status