Add a unique function.

docs/content/3.manual/manual.yml

@@ -542,6 +542,18 @@ sections:
             input: '[{"foo":1, "bar":10}, {"foo":3, "bar":100}, {"foo":1, "bar":1}]'
             output: ['[[{"foo":1, "bar":10}, {"foo":1, "bar":1}], [{"foo":3, "bar":100}]]']
 
+      - title: `unique`
+        body: |
+          
+          The `unique` function takes as input an array and produces
+          an array of the same elements, in sorted order, with
+          duplicates removed.
+
+        examples:
+          - program: 'unique'
+            input: '[1,2,5,3,5,3,1,3]'
+            output: ['[1,2,3,5]']
+
       - title: `contains`
         body: |