diff --git a/AUTHORS.adoc b/AUTHORS.adoc
index de89cd30..1e79d459 100644
--- a/AUTHORS.adoc
+++ b/AUTHORS.adoc
@@ -16,3 +16,4 @@
 - Johannes Kloos
 - Geoff Gole (@gsg)
 - Roma Sokolov (@little-arhat)
+- David Sheets (@dsheets)
diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index a70fd23e..85ecba8e 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -1,5 +1,12 @@
 = Changelog
 
+== 0.22
+
+- threads/CCLock: add `try_with_lock` to wrap `Mutex.try_lock`
+- Add `CCMultiSet.remove_all`
+- document errors in `CCIO` (close #86)
+- use the new qtest/qcheck
+
 == 0.21
 
 - (breaking) make default `start`/`stop` arguments empty in printers (#82)
diff --git a/README.adoc b/README.adoc
index 5028ce5a..08156bc8 100644
--- a/README.adoc
+++ b/README.adoc
@@ -195,8 +195,6 @@ Iterators:
 
 === String
 
-See http://cedeela.fr/~simon/software/containers/Containers_string[doc].
-
 In the module `Containers_string`:
 - `Levenshtein`: edition distance between two strings
 - `KMP`: Knuth-Morris-Pratt substring algorithm
@@ -204,8 +202,6 @@ In the module `Containers_string`:
 
 === Advanced
 
-See http://cedeela.fr/~simon/software/containers/Containers_advanced[doc].
-
 In the module `Containers_advanced`:
 - `CCLinq`, high-level query language over collections
 - `CCCat`, a few categorical structures
diff --git a/_oasis b/_oasis
index ba273d42..40bbc4d4 100644
--- a/_oasis
+++ b/_oasis
@@ -1,6 +1,6 @@
 OASISFormat: 0.4
 Name:        containers
-Version:     0.21
+Version:     0.22
 Homepage:    https://github.com/c-cube/ocaml-containers
 Authors:     Simon Cruanes
 License:     BSD-2-clause
diff --git a/opam b/opam
index d1bb2e04..cb986615 100644
--- a/opam
+++ b/opam
@@ -40,8 +40,6 @@ depopts: [
 ]
 conflicts: [
   "sequence" { < "0.5" }
-  "qtest" { < "2.2" }
-  "qcheck"
 ]
 tags: [ "stdlib" "containers" "iterators" "list" "heap" "queue" ]
 homepage: "https://github.com/c-cube/ocaml-containers/"
diff --git a/setup.ml b/setup.ml
index 657cf17a..e8ca7fc9 100644
--- a/setup.ml
+++ b/setup.ml
@@ -1,7 +1,7 @@
 (* setup.ml generated for the first time by OASIS v0.4.4 *)
 
 (* OASIS_START *)
-(* DO NOT EDIT (digest: 5a4456e803f365ce3410624a599bc76f) *)
+(* DO NOT EDIT (digest: 0f90f90895c33b40b6535d64f1d40619) *)
 (*
    Regenerated by OASIS v0.4.7
    Visit http://oasis.forge.ocamlcore.org for more information and
@@ -7049,7 +7049,7 @@ let setup_t =
        {
           oasis_version = "0.4";
           ocaml_version = Some (OASISVersion.VGreaterEqual "4.00.1");
-          version = "0.21";
+          version = "0.22";
           license =
             OASISLicense.DEP5License
               (OASISLicense.DEP5Unit
@@ -9689,7 +9689,7 @@ let setup_t =
        };
      oasis_fn = Some "_oasis";
      oasis_version = "0.4.7";
-     oasis_digest = Some "v\210\236\240,\180C\195X\143\011\\\217Y\180\206";
+     oasis_digest = Some "SzM\176?\172Sx\202\184\017a\207;\027f";
      oasis_exec = None;
      oasis_setup_args = [];
      setup_update = false
diff --git a/src/core/CCIO.mli b/src/core/CCIO.mli
index eee9682d..fb91a491 100644
--- a/src/core/CCIO.mli
+++ b/src/core/CCIO.mli
@@ -46,6 +46,7 @@ val with_in : ?mode:int -> ?flags:open_flag list ->
 (** Open an input file with the given optional flag list, calls the function
     on the input channel. When the function raises or returns, the
     channel is closed.
+    @raise Sys_error in case of error (same as {!open_in} and {!close_in})
     @param flags opening flags (default [[Open_text]]). [Open_rdonly] is used in any cases *)
 
 val read_chunks : ?size:int -> in_channel -> string gen
@@ -77,12 +78,14 @@ val with_out : ?mode:int -> ?flags:open_flag list ->
                string -> (out_channel -> 'a) -> 'a
 (** Same as {!with_in} but for an output channel
     @param flags opening flags (default [[Open_creat; Open_trunc; Open_text]]).
+    @raise Sys_error in case of error (same as {!open_out} and {!close_out})
     [Open_wronly] is used in any cases *)
 
 val with_out_a : ?mode:int -> ?flags:open_flag list ->
                   string -> (out_channel -> 'a) -> 'a
 (** Similar to {!with_out} but with the [[Open_append; Open_creat; Open_wronly]]
-    flags activated, to append to the file *)
+    flags activated, to append to the file.
+    @raise Sys_error in case of error (same as {!open_out} and {!close_out}) *)
 
 val write_line : out_channel -> string -> unit
 (** Write the given string on the channel, followed by "\n" *)
@@ -102,6 +105,7 @@ val with_in_out : ?mode:int -> ?flags:open_flag list ->
                   string -> (in_channel -> out_channel -> 'a) -> 'a
 (** Combines {!with_in} and {!with_out}.
     @param flags opening flags (default [[Open_creat]])
+    @raise Sys_error in case of error
     @since 0.12 *)
 
 (** {2 Misc for Generators} *)
@@ -145,7 +149,7 @@ module File : sig
   (** [remove_exn path] tries to remove the file at [path] from the
       file system.
 
-      {b Raises} [Sys_error] if there is no file at [path].
+      @raise Sys_error if there is no file at [path] or access rights are wrong.
       @since 0.8 *)
 
   val remove : t -> unit or_error
@@ -159,11 +163,13 @@ module File : sig
   val read_dir : ?recurse:bool -> t -> t gen
   (** [read_dir d] returns a sequence of files and directory contained
       in the directory [d] (or an empty stream if [d] is not a directory)
+      @raise Sys_error in case of error (e.g. permission denied)
       @param recurse if true (default [false]), sub-directories are also
         explored *)
 
   val read_exn : t -> string
   (** Read the content of the given file, or raises some exception
+      @raise Sys_error in case of error
       @since 0.16 *)
 
   val read : t -> string or_error
@@ -172,6 +178,7 @@ module File : sig
 
   val append_exn : t -> string -> unit
   (** Append the given string into the given file, possibly raising
+      @raise Sys_error in case of error
       @since 0.16 *)
 
   val append : t -> string -> unit or_error
@@ -180,6 +187,7 @@ module File : sig
 
   val write_exn : t -> string -> unit
   (** Write the given string into the given file, possibly raising
+      @raise Sys_error in case of error
       @since 0.16 *)
 
   val write : t -> string -> unit or_error
@@ -192,7 +200,8 @@ module File : sig
   (** Similar to {!read_dir} (with [recurse=true]), this function walks
       a directory recursively and yields either files or directories.
       Is a file anything that doesn't satisfy {!is_directory} (including
-      symlinks, etc.) *)
+      symlinks, etc.)
+      @raise Sys_error in case of error (e.g. permission denied) during iteration *)
 
   val show_walk_item : walk_item -> string
 
@@ -204,5 +213,6 @@ module File : sig
       After [f] returns, the file is deleted. Best to be used in
       combination with {!with_out}.
       See {!Filename.temp_file}
+      @raise Sys_error in case of error
       @since 0.17 *)
 end
diff --git a/src/core/META b/src/core/META
index 5a4f08ce..d39e8bdb 100644
--- a/src/core/META
+++ b/src/core/META
@@ -1,6 +1,6 @@
 # OASIS_START
-# DO NOT EDIT (digest: 2366633cbce20e67d6074e01f20927dc)
-version = "0.21"
+# DO NOT EDIT (digest: a16455ed7ae2b2e8c42add2f54bd446e)
+version = "0.22"
 description = "A modular standard library focused on data structures."
 requires = "bytes result"
 archive(byte) = "containers.cma"
@@ -9,7 +9,7 @@ archive(native) = "containers.cmxa"
 archive(native, plugin) = "containers.cmxs"
 exists_if = "containers.cma"
 package "unix" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "bytes unix"
  archive(byte) = "containers_unix.cma"
@@ -20,7 +20,7 @@ package "unix" (
 )
 
 package "top" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires =
  "compiler-libs.common containers containers.data containers.bigarray containers.string containers.unix containers.sexp containers.iter"
@@ -32,7 +32,7 @@ package "top" (
 )
 
 package "thread" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "containers threads"
  archive(byte) = "containers_thread.cma"
@@ -43,7 +43,7 @@ package "thread" (
 )
 
 package "string" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "bytes"
  archive(byte) = "containers_string.cma"
@@ -54,7 +54,7 @@ package "string" (
 )
 
 package "sexp" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "bytes"
  archive(byte) = "containers_sexp.cma"
@@ -65,7 +65,7 @@ package "sexp" (
 )
 
 package "iter" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  archive(byte) = "containers_iter.cma"
  archive(byte, plugin) = "containers_iter.cma"
@@ -75,7 +75,7 @@ package "iter" (
 )
 
 package "io" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "bytes"
  archive(byte) = "containers_io.cma"
@@ -86,7 +86,7 @@ package "io" (
 )
 
 package "data" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "bytes"
  archive(byte) = "containers_data.cma"
@@ -97,7 +97,7 @@ package "data" (
 )
 
 package "bigarray" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "containers bigarray bytes"
  archive(byte) = "containers_bigarray.cma"
@@ -108,7 +108,7 @@ package "bigarray" (
 )
 
 package "advanced" (
- version = "0.21"
+ version = "0.22"
  description = "A modular standard library focused on data structures."
  requires = "containers sequence"
  archive(byte) = "containers_advanced.cma"
diff --git a/src/data/CCMultiSet.ml b/src/data/CCMultiSet.ml
index 3b0bf680..c1f0c687 100644
--- a/src/data/CCMultiSet.ml
+++ b/src/data/CCMultiSet.ml
@@ -33,6 +33,10 @@ module type S = sig
       @raise Invalid_argument if [n < 0]
       @since 0.6 *)
 
+  val remove_all : t -> elt -> t
+  (** [remove_all set x] removes all occurrences of [x] from [set]
+      @since 0.22 *)
+
   val update : t -> elt -> (int -> int) -> t
   (** [update set x f] calls [f n] where [n] is the current multiplicity
       of [x] in [set] ([0] to indicate its absence); the result of [f n]
@@ -136,6 +140,8 @@ module Make(O : Set.OrderedType) = struct
 
   let remove ms x = remove_mult ms x 1
 
+  let remove_all ms x = M.remove x ms
+
   let update ms x f =
     let n = count ms x in
     match f n with
diff --git a/src/data/CCMultiSet.mli b/src/data/CCMultiSet.mli
index 8dcb6b9a..2f060dc2 100644
--- a/src/data/CCMultiSet.mli
+++ b/src/data/CCMultiSet.mli
@@ -33,6 +33,10 @@ module type S = sig
       @raise Invalid_argument if [n < 0]
       @since 0.6 *)
 
+  val remove_all : t -> elt -> t
+  (** [remove_all set x] removes all occurrences of [x] from [set]
+      @since 0.22 *)
+
   val update : t -> elt -> (int -> int) -> t
   (** [update set x f] calls [f n] where [n] is the current multiplicity
       of [x] in [set] ([0] to indicate its absence); the result of [f n]
diff --git a/src/threads/CCLock.ml b/src/threads/CCLock.ml
index cd9aa456..c1f0bf14 100644
--- a/src/threads/CCLock.ml
+++ b/src/threads/CCLock.ml
@@ -35,6 +35,18 @@ let with_lock l f =
   assert_equal 10 (get l)
 *)
 
+let try_with_lock l f =
+  if Mutex.try_lock l.mutex
+  then
+    try
+      let x = f l.content in
+      Mutex.unlock l.mutex;
+      Some x
+    with e ->
+      Mutex.unlock l.mutex;
+      raise e
+  else None
+
 module LockRef = struct
   type 'a t = 'a lock
   let get t = t.content
diff --git a/src/threads/CCLock.mli b/src/threads/CCLock.mli
index 75e4b07c..354e47c0 100644
--- a/src/threads/CCLock.mli
+++ b/src/threads/CCLock.mli
@@ -18,6 +18,12 @@ val with_lock : 'a t -> ('a -> 'b) -> 'b
     the lock [l], in a critical section. If [f x] fails, [with_lock l f]
     fails too but the lock is released *)
 
+val try_with_lock : 'a t -> ('a -> 'b) -> 'b option
+(** [try_with_lock l f] runs [f x] in a critical section if [l] is not
+    locked. [x] is the value protected by the lock [l]. If [f x]
+    fails, [try_with_lock l f] fails too but the lock is released
+    @since 0.22 *)
+
 (** Type allowing to manipulate the lock as a reference
     @since 0.13 *)
 module LockRef : sig
@@ -48,7 +54,8 @@ val mutex : _ t -> Mutex.t
 (** Underlying mutex *)
 
 val get : 'a t -> 'a
-(** Get the value in the lock. The value that is returned isn't protected! *)
+(** Atomically get the value in the lock. The value that is returned
+    isn't protected! *)
 
 val set : 'a t -> 'a -> unit
 (** Atomically set the value