The tt(IFMACRO) function tests whether its first argument is the name of a
macro. If the name is the name of a macro, the second argument is evaluated,
else, the third argument is evaluated. All three arguments (the name, the true
list and the false list) must be present; though the true list and/or the
false list may be empty.

    Example:
        verb(
    IFMACRO(nested)(\ 
        `nested' is the name of a macro
    )(\ 
        There is no macro named `nested'
    )
        )
    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. 

