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if  test-form then-form [else-form] ⇒  {result}*
Test-form—a form.
Then-form—a form.
Else-form—a form. The default is nil.
results—if the test-form yielded true, the values returned by the then-form; otherwise, the values returned by the else-form.
if allows the execution of a form to be dependent on a single test-form.
First test-form is evaluated. If the result is true, then then-form is selected; otherwise else-form is selected. Whichever form is selected is then evaluated.
 (if t 1) ⇒  1
 (if nil 1 2) ⇒  2 
 (defun test ()
   (dolist (truth-value '(t nil 1 (a b c)))
     (if truth-value (print 'true) (print 'false))
     (prin1 truth-value))) ⇒  TEST
 (test)
 |>  TRUE T
 |>  FALSE NIL
 |>  TRUE 1
 |>  TRUE (A B C)
⇒  NIL
(if test-form then-form else-form) ≡ (cond (test-form then-form) (t else-form))