Implement the 'add' builtin promised by the docs' examples.

docs/content/3.manual/manual.yml

@@ -321,6 +321,43 @@ sections:
             input: '[[1,2], "string", {"a":2}, null]'
             output: [2, 6, 1, 0]
 
+      - title: `map(x)`
+        body: |
+
+          For any filter `x`, `map(x)` will run that filter for each
+          element of the input array, and produce the outputs a new
+          array. `map(.+1)` will increment each element of an array of numbers.
+
+          `map(x)` is equivalent to `[.[] | x]`. In fact, this is how
+          it's defined.
+
+        examples:
+          - program: 'map(.+1)'
+            input: '[1,2,3]'
+            output: ['[2,3,4]']
+
+      - title: `add`
+        body: |
+
+          The filter `add` takes as input an array, and produces as
+          output the elements of the array added together. This might
+          mean summed, concatenated or merged depending on the types
+          of the elements of the input array - the rules are the same
+          as those for the `+` operator (described above).
+
+          If the input is an empty array, `add` returns `null`.
+
+        examples:
+          - program: add
+            input: '["a","b","c"]'
+            output: ["abc"]
+          - program: add
+            input: '[1, 2, 3]'
+            output: [6]
+          - program: add
+            input: '[]'
+            output: ["null"]
+            
       - title: `tonumber`
         body: |