OCaml code snippets proposed as CC-BY-SA 4.0 candidates

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A simple functional queue (FIFO datatype)
(This code is outrageous plagiarism of the excellent Chris Okasaki book, page 43)
(This is common knowledge, i don’t see why this code could not be CC-BY-SA 4.0)

module Queue
:
sig
   type +'a t
   val empty : 'a t
   val member : 'a t -> 'a
   val add : 'a -> 'a t -> 'a t
   val remove : 'a t -> 'a t
end
=
struct
   type +'a t =
      {front:'a list;rear:'a list}
   let empty =
      {front=[];rear=[]}
   let member {front;_} =
      match front with
      | [] -> failwith "Queue.member"
      | h::_ -> h
   let queue front rear =
      match front with
      | [] -> {front=List.rev rear;rear=[]}
      | _ -> {front;rear}     
   let add n {front;rear} =
      queue front (n::rear)
   let remove {front;rear} =
      match front with
      | [] -> failwith "Queue.remove"
      | _::t -> queue t rear
end