Next: , Up: if  if in more detail  An if expression written in
Lisp does not use the word `then'; the test and the action are
the second and third elements of the list whose first element is
if. Nonetheless, the test part of an if expression is often
called the if-part and the second argument is often called the
then-part.  Also, when an if expression is written, the true-or-
false-test is usually written on the same line as the symbol if,
but the action to carry out if the test is true, the “then-
part”, is written on the second and subsequent lines. This makes
the if expression easier to read.       (if true-or-false-test
The true-or-false-test will be an expression that is evaluated
by the Lisp interpreter.  Here is an example that you can
evaluate in the usual manner. The test is whether the number 5
is greater than the number 4. Since it is, the message ‘5 is
greater than 4!’ will be printed.       (if (> 5 4)
; if-part          (message "5 is greater than 4!"))   ; then-
part