fix error

src/Reader.ml

@@ -10,6 +10,7 @@ sig
   val read : unit -> env
   val scope : (env -> env) -> (unit -> 'a) -> 'a
   val run : env:env -> (unit -> 'a) -> 'a
+  val register_printer : ([`Read] -> string option) -> unit
 end
 
 module Make (P : Param) =

src/Sequencer.ml

@@ -9,6 +9,7 @@ sig
 
   val yield : elt -> unit
   val run : (unit -> unit) -> elt Seq.t
+  val register_printer : ([`Yield of elt] -> string option) -> unit
 end
 
 module Make (P : Param) =

src/State.ml

@@ -11,6 +11,7 @@ sig
   val set : state -> unit
   val modify : (state -> state) -> unit
   val run : init:state -> (unit -> 'a) -> 'a
+  val register_printer : ([`Get | `Set of state] -> string option) -> unit
 end
 
 module Make (P : Param) =

src/UniqueID.ml

@@ -21,6 +21,7 @@ sig
   val retrieve : id -> elt
   val export : unit -> elt Seq.t
   val run : ?init:elt Seq.t -> (unit -> 'a) -> 'a
+  val register_printer : ([`Register of elt | `Retrieve of id | `Export] -> string option) -> unit
 end
 
 module Make (P : Param) =