Conditional Compilation: Directives and Symbols
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   Directive  Meaning
   DEFINE     Defines a conditional symbol
   ELSE       Compiles or ignores a portion of source text
   ENDIF      Ends the conditional compilation
   IFDEF      Compiles source text if Name is defined
   IFNDEF     Compiles source text if Name is NOT defined
   IFOPT      Compiles source text if a compiler switch is in a specified
              state (+ or -)
   UNDEF      Undefines a previously defined conditional symbol

   Symbol     Meaning
   CPU86      CPU belongs to 80x86 processor family
   CPU87      Defined only if an 80x87 numeric coprocessor is present at
              compile time
   DPMI       Indicates operating environment is DOS protected mode
   MSDOS      Indicates operating system is real-mode MS-DOS or PC-DOS
   VER70      Turbo Pascal version number
   WINDOWS    Indicates operating enviroment is MS-Windows

 Conditional compilation constructs