Add specifications for set functions (#25)

Author: kgryte

spec/API_specification/index.rst

@@ -16,5 +16,6 @@ API specification
    creation_functions
    elementwise_functions
    statistical_functions
+   linear_algebra_functions
    searching_functions
-   linear_algebra_functions
+   set_functions

spec/API_specification/set_functions.md

@@ -0,0 +1,61 @@
+# Set Functions
+
+> Array API specification for creating and operating on sets.
+
+A conforming implementation of the array API standard must provide and support the following functions adhering to the following conventions.
+
+-   Positional parameters must be [positional-only](https://www.python.org/dev/peps/pep-0570/) parameters. Positional-only parameters have no externally-usable name. When a function accepting positional-only parameters is called, positional arguments are mapped to these parameters based solely on their order.
+-   Optional parameters must be [keyword-only](https://www.python.org/dev/peps/pep-3102/) arguments.
+-   Unless stated otherwise, functions must support the data types defined in :ref:`data-types`.
+
+<!-- NOTE: please keep the functions in alphabetical order -->
+
+### <a name="unique" href="#unique">#</a> unique(x, /, *, return_counts=False, return_index=False, return_inverse=False, sorted=True)
+
+Returns the unique elements of an input array `x`.
+
+#### Parameters
+
+-   **x**: _&lt;array&gt;_
+
+    -   input array. If `x` has more than one dimension, the function must flatten `x` and return the unique elements of the flattened array.
+
+-   **return_counts**: _bool_
+
+    -   If `True`, the function must also return the number of times each unique element occurs in `x`. Default: `False`.
+
+-   **return_index**: _bool_
+
+    -   If `True`, the function must also return the indices (first occurrences) of `x` that result in the unique array. Default: `False`.
+
+-   **return_inverse**: _bool_
+
+    -   If `True`, the function must also return the indices of the unique array that reconstruct `x`. Default: `False`.
+
+-   **sorted**: _bool_
+
+    -   If `True`, the function must sort the unique elements in ascending order before returning as output. If `False`, the function must sort the unique elements in the same order that they occur in `x`. Default: `False`.
+
+        _TODO: sort order needs discussion. See [gh-40](https://github.com/data-apis/array-api/issues/40)_
+
+#### Returns
+
+-   **out**: _Union\[ &lt;array&gt;, Tuple\[ &lt;array&gt;, ... ] ]_
+
+    -   if `return_counts`, `return_index`, and `return_inverse` are all `False`, an array containing the set of unique elements in `x`; otherwise, a tuple containing two or more of the following arrays (in order):
+
+        -   **unique**: _&lt;array&gt;_
+
+            -   an array containing the set of unique elements in `x`.
+
+        -   **indices**: _&lt;array&gt;_
+
+            -   an array containing the indices (first occurrences) of `x` that result in `unique`.
+
+        -   **inverse**: _&lt;array&gt;_
+
+            -   an array containing the indices of `unique` that reconstruct `x`.
+
+        -   **counts**: _&lt;array&gt;_
+
+            -   an array containing the number of times each unique element occurs in `x`.