The tt(IFDEF) macro tests for the logical true value of the argument in its
first parameter list. If `true', the second parameter list is evaluated, else,
the third parameter list is evaluated. All three parameter lists (the
variable, the true-list and the false-list) must be present; though the
true-list and/or the false-list may be empty parameter lists.

The variable in the first parameter list evaluates the macro to `true' when 
the first argument is:

startit()

    it() a built-in macro,
   
    it() a user-defined macro (see tt(DEFINEMACRO), section ref(definemacro)),
    
    it() a logical symbol (see tt(DEFINESYMBOL), section ref(definesymbol)),
   
    it() a counter (see tt(NEWCOUNTER), section ref(newcounter)).
    
endit()

For an example, see the description of tt(DEFINESYMBOL), section 
ref(definesymbol). 
