Add tests.

src/Sequence.ml

@@ -1321,6 +1321,14 @@ let sample k seq =
     if !i < k then Array.sub a 0 (!i + 1)
     else a
 
+(*$QR
+  Q.(pair (list int) (1 -- 20)) (fun (l, n) ->
+    let seq = of_list l in
+    let a = sample n seq in
+    (Array.for_all (fun x -> exists ((=) x) seq) a)
+    && (Array.length a = Pervasives.min (length seq) n) )
+*)
+
 (** {2 Infix functions} *)
 
 module Infix = struct