foo.c: In function 'f':
foo.c:13:18: error: expected expression before '?' token
  return f(n-k,k)/?????????1??????-1/+f(n-1,k-1)/????????????1???k-1?/;
                  ^
foo.c:13:32: warning: trigraph ??- ignored, use -trigraphs to enable [-Wtrigraphs]
  return f(n-k,k)/?????????1??????-1/+f(n-1,k-1)/????????????1???k-1?/;
 ^
foo.c:14:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^
foo.c: In function 'main':
foo.c:21:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^