#ifdef FOO_LIBRARY
#error Unexpected FOO_LIBRARY
#endif
#ifdef BAR_USE_BANG
# ifndef BANG_LIBRARY
# error Expected BANG_LIBRARY
# endif
# include "bang.h"
#else
# ifdef BANG_LIBRARY
# error Unexpected BANG_LIBRARY
# endif
#endif
#include "bar.h"
int bar()
{
#ifdef BAR_USE_BANG
return bang();
#else
return 0;
#endif
}