#define true false
  #define false true

  #define true 0
  #define false !true

  #define if while
  #define continue break

  #define true false
  #define false true

The consequence of this rule is that cycles of #defines don't do anything. Edit: Well, that's only true if there aren't other symbols introduced during expansion. Here's a counterexample that prints 3:

   #include <stdio.h>

   int x = 1;

   #define x 1 + y
   #define y 1 + x

   int main(int argc, char *argv[])
   {
     printf("%d\n",x);
     return 0;
   }

First make a file a.c that has in it:

#define true false

#define false true

int A = true;

int B = false;

then from the command line, do (with the Microsoft compiler): D:\>cl -E a.c

int A = true;

int B = false;

Now with gcc (I'm using cygwin's gcc): D:\>sh -c "gcc -E a.c"

int A = true;

int B = false;

I've stripped the other stuff.

  #define TRUE random()%2

(While being quite funny, mind you!)

The image halted immediately.