Added [Seq.concat].

src/Seq.ml

@@ -26,3 +26,10 @@ let rec elements xs accu =
 
 let elements xs =
   elements xs []
+
+let rec concat xss =
+  match xss with
+  | [] ->
+      empty
+  | xs :: xss ->
+      append xs (concat xss)

src/Seq.mli

@@ -7,3 +7,4 @@ val empty: 'a seq
 val singleton: 'a -> 'a seq
 val append: 'a seq -> 'a seq -> 'a seq
 val elements: 'a seq -> 'a list
+val concat: 'a seq list -> 'a seq