write List.flat_map

lib/server/import.ml

@@ -16,4 +16,15 @@ module Result_syntax = struct
   let ( let+ ) r f = Result.map f r
 end
 
+module List = struct
+  include List
+
+  let flat_map f xs =
+    let rec iter acc = function
+      | [] -> rev acc
+      | x :: xs -> iter (rev_append (f x) acc) xs
+    in
+    iter [] xs
+end
+
 include (val Logging.logs "Irc")