8.2. Deciding what gets included

Including or discarding parts of the a source file is done by using #if directives, just like in the script language. 

// Simple if block 
#if TEST
    inc $d020
#endif       // <- Use an endif to close this if block 

// You can also use else
#if A
    .print "A is defined"
#else
    .print "A is not defined"
#endif

Since the source isn't passed on to the main parser, you can write anything inside an untaken if, and it will still compile. 

#undef UNDEFINED_SYMBOL
#if UNDEFINED_SYMBOL
    Here we can write anything since it will never be seen by the main parser... 
#endif

#elif is the combination of an #else and an #if. It can be used like this: 

#if X
    .print "X"
#elif Y
    .print "Y"
#elif Z
    .print "Z"
#else
    .print "Not X, Y and Z"
#endif

The #if blocks can be nested:

#if A
    #if B
        .print "A and B"   
    #endif 
#else
    #if X
        .print "not A and X"
    #elif Y
        .print "not A and Y" 
    #endif
#endif

The indentations doesn't change anything, its just to make the code easier to read.

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