
The macro tt(IFEMPTY) expects three arguments: a symbol, a true-list and a
false-list. The macro evaluates to the true-list if the symbol is an empty
string; otherwise, it evaluates to the false-list.

A trivial example is the following:

verb(\
IFEMPTY(something) \ 
    ("something" is supposedly nothing ?!?")
    ("something" is not an empty string))
    
In the same way, tt(IFEMPTY) can be used to test whether a macro expands to a
non-empty string. A more elaborate example follows below. Say you want to
define a tt(bookref) macro to typeset information about an author, a book
title and about the publisher. The publisher information may be absent, the
macro then typesets tt(unknown):

verb(\
DEFINEMACRO(bookref)(3)(\ 
Author(s):      ARG1
Title:          ARG2
Published by:   IFEMPTY(ARG3)(Unknown)(ARG3)))

Using the macro, as in:

verb(\
bookref(Helmut Leonhardt)
       (Histologie, Zytologie und Microanatomie des Menschen)
       ())
       
would of course supply tt(Unknown) in the tt(Published by:) line.
