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Examples of filtering
Expression

 Value

(filter 
  even?
 '(1 2 3 4 5))
 
(2 4)
(filter 
  (negate even?)
  '(1 2 3 4 5))
 
(1 3 5)
(ol 
 (map li
  (filter string?
   (list 
     1 'a "First" "Second" 3))))
  1. First
  2. Second
Same as above 
<ol>
  <li>First</li> <li>Second</li>
</ol>