IsDefined MACRO symbol       (Macro Function)

   Determines if a symbol has been defined.
   This is a macro function so the parameter must be enclosed in parentheses.
   (Required for Chapter 10 example, page 367-368)

Parameters:

   ( symbol ) - An identifier.

Returns:

    -1 if a symbol has been defined, or else return 0.

Example: 

      IF IsDefined( RealMode )
         mov  ax,@data
         mov  ds,ax
         mov  es,ax
      ENDIF
    If identifier RealMode has not been defined, no code is generated.

    If RealMode has been defined, for example, by placing RealMode EQU 1 in the source code,
or by placing -DRealMode=1 on the ML command line, then the following code is generated: 
         mov ax,@data
         mov  ds,ax
         mov  es,ax
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