tt(IFZERO) expects three arguments. If the first argument is zero (0)
the function expands to the true list (the second argument). Otherwise it
expands to the false list (the third argument).

    The first argument of tt(IFZERO) should be an integral numeric value. Its
value is determined as follows:
    itemization(
    it() If the argument starts with an integral numerical value, that value
is the value of the argument.
    it() If the argument is the name of a counter, the counter's value is the
value of the argument
    it() Otherwise, the first arguments evaluates as 0, and the false list is
used.
    )

    Note that, starting with Yodl version 2.00 the first argument is not
evaluated. So tt(COUNTERVALUE(somecounter)) always evaluates as 0. If the
value of a counter is required, simply provide its name as the first argument
of the tt(IFZERO) function.
    
    Example:
        verb(
    DEFINEMACRO(environment)(2)(\ 
        IFZERO(ARG2)(\ 
            NOEXPAND(\end{ARG1})\ 
        )(\  
            NOEXPAND(\begin{ARG1})\ 
        )\ 
    )    
        )
    Such a macro may be used as follows:
        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.

