#define BEAR 23

#ifdef BEAR
The bear went over the mountain.
#endif

#ifdef GOLD
There's gold in them thar hills.
#else
There ain't no gold in them thar hills.
#endif

#define GOLD 17

#ifdef GOLD
There really is gold in them thar hills.
#elif BEAR
The bear still going over the mountain -- but only if there is no gold.
#endif

#undef GOLD

#ifdef GOLD
There really is gold in them thar hills.
#elif BEAR
The bear still going over the mountain -- but only because there is no gold.
#endif

#if defined(BEAR) && BEAR%10==3
The number of bears ends with a 3
#endif