The tt(IFZERO) macro expects three parameter lists, just as the other
tt(IF...) macros: a symbol, a true-list and a false-list. 

The first argument defines whether the whole macro expands to the true-list or
to the false-list. The first argument may be:

itemize(

    it() Nothing (empty): the macro expands to the true-list.

    it() A number: when zero, the macro expands to the true-list.
    
    it() The name of a counter: when the value of the counter is zero, the
    macro expands to the true-list. 
    
    it() Anything else: the macro expands to the false-list.
    
)

You can use tt(COUNTERVALUE(somecounter)) as the argument to tt(IFZERO), it is
treated in the same manner as the corresponding counter name. 

Example: The tt(IFZERO) macro offers a very simple way to pass a flag-argument
(an on/off switch) to a macro. E.g., in LaTeX you start environments with
tt(\begin{environment}) and end them with tt(\end{environment});
tt(environment) being e.g., tt(center), tt(flushright), tt(flushleft). A
possible meta-macro for the environments might be:

verb(\
DEFINEMACRO(environment)(2)(\ 
    IFZERO(ARG2)\ 
        NOEXPAND(\end{ARG1})\ 
        NOEXPAND(\begin{ARG1})))
        
Such a macro may be used as:

verb(\
environment(center)(1)
Now comes centered text.
environment(center)(0))

which would of course lead to tt(\begin) and tt(\end{center}). The numeric
second argument is used here as a on/off switch.
