Basic30 Language Reference
#ifndef ... #endif

Tests to see if a definition is not made.

#ifndef ifdef_expression
[false statements]
[#else
[true statements]]
#endif

ifdef_expression: Required one or more constants to check for. The characters & and | are used to check for more than one constant.
true statements: Optional statements to be compiled when ifndef_expression is TRUE.
false statements: Optional statements to be compiled when ifndef_expression is FALSE

Examples

#define 30F6014
#define debugVersion

#ifndef 30F6014
    ' Statements here will not be compiled because 30F6014 is defined
#endif

#ifndef 30F6014 & releaseVersion
    ' Statements here will be compiled because releaseVersion is not defined
#endif

#ifndef debugVersion | releaseVersion
    ' Statements here will not be compiled because debugVersion is defined
#endif

Also See: #define, #undefine, #ifdef