The tt(IFDEF) function tests for the definition status of the argument in its
first argument. If it is a defined entity, the second argument is evaluated,
else, the third argument is evaluated. All three arguments (the entity, the
true list and the false list) must be present; though the true list and/or the
false list may be empty.

    The true list is evaluated if the first argument is the name of:
    itemization(
    it() a built-in function, or
    it() a character table, or
    it() a counter, or
    it() a no-user-macro symbol, or
    it() a symbol, or
    it() a user-defined macro, or
    )
    Example:
        verb(
    IFDEF(someName)(\ 
        `someName' is a defined entity
    )(\ 
        `someName is not defined.
    )
        )
    Please note the preferred layout: The first argument immediately follows
the function name, then the second argument (the em(true list)) is indented,
as is the em(false list). The layout closely follows the preferred layout of
tt(if-else) statements of many programming languages. 
